Thursday, August 18, 2022


From Wikipedia, the free encyclopedia

In order from the top: An electric locomotive pulling a passenger train in Russia, a rack railway in Switzerland, diesel locomotives pulling a freight train in New Zealand, a monorail in Germany, a metro train in India, a restored steam locomotive in the U.S.

In rail transport, a train is a series of connected vehicles that run along a railway track and transport people or freight. The word train comes from the Old French trahiner, derived from the Latin trahere meaning "to pull, to draw". Trains are typically pulled or pushed by locomotives (often known simply as "engines"), though some are self-propelled, such as multiple units. Passengers and cargo are carried in railroad cars, also known as wagons. Trains are designed to a certain gauge, or distance between rails. Most trains operate on steel tracks with steel wheels, the low friction of which makes them more efficient than other forms of transport.

Trains have their roots in wagonways, which used railway tracks and were powered by horses or pulled by cables. Following the invention of the steam locomotive in the United Kingdom in 1804, trains rapidly spread around the world, allowing freight and passengers to move over land faster and cheaper than ever possible before. Rapid transit and trams were first built in the late 1800s to transport large numbers of people in and around cities. Beginning in the 1920s, and accelerating following World War II, diesel and electric locomotives replaced steam as the means of motive power. Following the development of cars, trucks, and extensive networks of highways which offered greater mobility, as well as faster airplanes, trains declined in importance and market share, and many rail lines were abandoned. The spread of buses led to the closure of many rapid transit and tram systems during this time as well.

Since the 1970s, governments, environmentalists, and train advocates have promoted increased use of trains due to their greater fuel efficiency and lower greenhouse gas emissions compared to other modes of land transport. High-speed rail, first built in the 1960s, has proven competitive with cars and planes over short to medium distances. Commuter rail has grown in importance since the 1970s as an alternative to congested highways and a means to promote development, as has light rail in the 21st century. Freight trains remain important for the transport of bulk commodities such as coal and grain, as well as being a means of reducing road traffic congestion by freight trucks.

While conventional trains operate on relatively flat tracks with two rails, a number of specialized trains exist which are significantly different in their mode of operation. Monorails operate on a single rail, while funiculars and rack railways are uniquely designed to traverse steep slopes. Experimental trains such as high speed maglevs, which use magnetic levitation to float above a guideway, are under development in the 2020s and offer higher speeds than even the fastest conventional trains. Development of trains which use alternative fuels such as natural gas and hydrogen is another 21st century development.


Early history

Stockton and Darlington special inaugural train 1825: six wagons of coal, directors coach, then people in wagons

Trains are an evolution of wheeled wagons running on stone wagonways, the earliest of which were built by Babylon circa 2,200 BCE. Starting in the 1500s, wagonways were introduced to haul material from mines; from the 1790s, stronger iron rails were introduced. Following early developments in the second half of the 1700s, in 1804 a steam locomotive built by British inventor Richard Trevithick powered the first ever steam train. Outside of coal mines, where fuel was readily available, steam locomotives remained untried until the opening of the Stockton and Darlington Railway in 1825. British engineer George Stephenson ran a steam locomotive named Locomotion No. 1 on this 40-kilometer (25-mile) long line, hauling over 400 passengers at up to 13 kilometers per hour (8 mph). The success of this locomotive, and Stephenson's Rocket in 1829, convinced many of the value in steam locomotives, and within a decade the stock market bubble known as "Railway Mania" started across the United Kingdom.

News of the success of steam locomotives quickly reached the United States, where the first steam railroad opened in 1829. American railroad pioneers soon started manufacturing their own locomotives, designed to handle the sharper curves and rougher track typical of the country's railroads.

The Union Pacific Big Boy locomotives represented the pinnacle of steam locomotive technology and power

The other nations of Europe also took note of British railroad developments, and most countries on the continent constructed and opened their first railroads in the 1830s and 1840s, following the first run of a steam train in France in late 1829. In the 1850s, trains continued to expand across Europe, with many influenced by or purchases of American locomotive designs. Other European countries pursued their own distinct designs. Around the world, steam locomotives grew larger and more powerful throughout the rest of the century as technology advanced.

Trains first entered service in South America, Africa, and Asia through construction by imperial powers, which starting in the 1840s built railroads to solidify control of their colonies and transport cargo for export. In Japan, which was never colonized, railroads first arrived in the early 1870s. By 1900, railroads were operating on every continent besides uninhabited Antarctica.

New technologies

Even as steam locomotive technology continued to improve, inventors in Germany started work on alternative methods for powering trains. Werner von Siemens built the first train powered by electricity in 1879, and went on to pioneer electric trams. Another German inventor, Rudolf Diesel, constructed the first diesel engine in the 1890s, though the potential of his invention to power trains was not realized until decades later. Between 1897 and 1903, tests of experimental electric locomotives on the Royal Prussian Military Railway in Germany demonstrated they were viable, setting speed records in excess of 160 kilometers per hour (100 mph).

The EMD FT set the stage for diesel locomotives to take over from steam.

Early gas powered "doodlebug" self-propelled railcars entered service on railroads in the first decade of the 1900s. Experimentation with diesel and gas power continued, culminating in the German "Flying Hamburger" in 1933, and the influential American EMD FT in 1939. These successful diesel locomotives showed that diesel power was superior to steam, due to lower costs, ease of maintenance, and better reliability. Meanwhile, Italy developed an extensive network of electric trains during the first decades of the 20th century, driven by that country's lack of significant coal reserves.

Dieselization and increased competition

World War II brought great destruction to existing railroads across Europe, Asia, and Africa. Following the war's conclusion in 1945, nations which had suffered extensive damage to their railroad networks took the opportunity provided by Marshall Plan funds (or economic assistance from the USSR and Comecon, for nations behind the Iron Curtain) and advances in technology to convert their trains to diesel or electric power. France, Russia, Switzerland, and Japan were leaders in adopting widespread electrified railroads, while other nations focused primarily on dieselization. By 1980, the majority of the world's steam locomotives had been retired, though they continued to be used in parts of Africa and Asia, along with a few holdouts in Europe and South America. China was the last country to fully dieselize, due to its abundant coal reserves; steam locomotives were used to haul mainline trains as late as 2005 in Inner Mongolia.

Trains began to face strong competition from automobiles and freight trucks in the 1930s, which greatly intensified following World War II. After the war, air transport also became a significant competitor for passenger trains. Large amounts of traffic shifted to these new forms of transportation, resulting in a widespread decline in train service, both freight and passenger. A new development in the 1960s was high-speed rail, which runs on dedicated rights of way and travels at speeds of 240 kilometers per hour (150 mph) or greater. The first high-speed rail service was the Japanese Shinkansen, which entered service in 1964. In the following decades, high speed rail networks were developed across much of Europe and Eastern Asia, providing fast and reliable service competitive with automobiles and airplanes. The first high-speed train in the Americas was Amtrak's Acela in the United States, which entered service in 2000.

China operates an extensive high speed rail network

To the present day

Towards the end of the 20th century, increased awareness of the benefits of trains for transport led to a revival in their use and importance. Freight trains are significantly more efficient than trucks, while also emitting far fewer greenhouse gas emissions per ton-mile; passenger trains are also far more energy efficient than other modes of transport. According to the International Energy Agency, "On average, rail requires 12 times less energy and emits 7-11 times less GHGs per passenger-km travelled than private vehicles and airplanes, making it the most efficient mode of motorised passenger transport. Aside from shipping, freight rail is the most energy-efficient and least carbon-intensive way to transport goods." As such, rail transport is considered an important part of achieving sustainable energy. Intermodal freight trains, carrying double-stack shipping containers, have since the 1970s generated significant business for railroads and gained market share from trucks. Increased use of commuter rail has also been promoted as a means of fighting traffic congestion on highways in urban areas.

Types and terminology

Trains can be sorted into types based on whether they haul passengers or freight (though mixed trains which haul both exist), by their weight (heavy rail for regular trains, light rail for lighter rapid transit systems), by their speed, and by what form of track they use. Conventional trains operate on two rails, but several other types of track systems are also in use around the world.


The railway terminology that is used to describe a train varies between countries. The two primary systems of terminology are International Union of Railways terms in much of the world, and Association of American Railroads terms in North America.

Trains are typically defined as one or more locomotives coupled together, with or without cars. A collection of rail vehicles may also be called a consist. A set of vehicles that are permanently or semi-permanently coupled together (such as the Pioneer Zephyr) is called a trainset. The term rolling stock is used to describe any kind of train vehicle.



US-style railroad truck (bogie) with journal bearings

Bogies, also known in North America as trucks, support the wheels and axles of trains. Trucks range from just one axle to as many as four or more. Two-axle trucks are in the widest use worldwide, as they are better able to handle curves and support heavy loads than single axle trucks.


Train vehicles are linked to one another by various systems of coupling. In much of Europe, India, and South America, trains primarily use buffers and chain couplers, while in the rest of the world knuckle couplers are used.


Because trains are heavy, powerful brakes are needed to slow or stop trains, and because steel wheels on steel rails have relatively low friction, brakes must be distributed among as many wheels as possible. Early trains could only be stopped by manually applied hand brakes, requiring workers to ride on top of the cars and apply the brakes when the train went downhill. Hand brakes are still used to park cars and locomotives, but the predominant braking system for trains globally is air brakes, invented in 1869 by George Westinghouse. Air brakes are applied at once to the entire train using air hoses.

Warning devices

This cab car includes a horn (top), a bell (top right), headlights (above the door), classification lights (red lights on side), and ditch lights (white lights on side)

For safety and communication, trains are equipped with bells, horns, and lights. Steam locomotives typically use steam whistles rather than horns. Other types of lights may be installed on locomotives and cars, such as classification lights, Mars Lights, and ditch lights.


Locomotives are in most cases equipped with cabs, also known as driving compartments, where a train driver controls the train's operation. They may also be installed on unpowered train cars known as cab or control cars, to allow for a train to operate with the locomotive at the rear.


Scheduling and dispatching

To prevent collisions or other accidents, trains are often scheduled, and almost always are under the control of train dispatchers. Historically, trains operated based on timetables; most passenger trains continue to operate based on fixed schedules, though freight trains may instead run on an as-needed basis, or when enough freight cars are available to justify running a train.


A number of maintenance vehicles at work on Metro-North Railroad

Simple repairs may be done while a train is parked on the tracks, but more extensive repairs will be done at a motive power depot. Similar facilities exist for repairing damaged or defective train cars. Maintenance of way trains are used to build and repair railroad tracks and other equipment.


Train drivers, also known as engineers, are responsible for operating trains. Conductors are in charge of trains and their cargo, and help passengers on passenger trains. Brakeman, also known as trainmen, were historically responsible for manually applying brakes, though the term is used today to refer to crew members who perform tasks such as operating switches, coupling and uncoupling train cars, and setting handbrakes on equipment. Steam locomotives require a fireman who is responsible for fueling and regulating the locomotive's fire and boiler. On passenger trains, other crew members assist passengers, such as chefs to prepare food, and service attendants to provide food and drinks to passengers. Other passenger train specific duties include passenger car attendants, who assist passengers with boarding and alighting from trains, answer questions, and keep train cars clean, and sleeping car attendants, who perform similar duties in sleeping cars.


A narrow gauge train in Austria

Around the world, various track gauges are in use for trains. In most cases, trains can only operate on tracks that are of the same gauge; where different gauge trains meet, it is known as a break of gauge. Standard gauge, defined as 1,435 mm (4 ft 8.5 in) between the rails, is the most common gauge worldwide, though both broad-gauge and narrow-gauge trains are also in use. Trains also need to fit within the loading gauge profile to avoid fouling bridges and lineside infrastructure with this being a potential limiting factor on loads such as intermodal container types that may be carried.


Most derailments, such as this one in Switzerland, are minor and do not cause injuries or damage.

Train accidents sometimes occur, including derailments (when a train leaves the tracks) and train wrecks (collisions between trains). Accidents were more common in the early days of trains, when railway signal systems, centralized traffic control, and failsafe systems to prevent collisions were primitive or did not yet exist. To prevent accidents, systems such as automatic train stop are used; these are failsafe systems that apply the brakes on a train if it passes a red signal and enters an occupied block, or if any of the train's equipment malfunctions. More advanced safety systems, such as positive train control, can also automatically regulate train speed, preventing derailments from entering curves or switches too fast.

Modern trains have a very good safety record overall, comparable with air travel. In the United States between 2000 and 2009, train travel averaged 0.43 deaths per billion passenger miles traveled. While this was higher than that of air travel at 0.07 deaths per billion passenger miles, it was also far below the 7.28 deaths per billion passenger miles of car travel. In the 21st century, several derailments of oil trains caused fatalities, most notably the Canadian Lac-Mégantic rail disaster in 2013 which killed 47 people and leveled much of the town of Lac-Mégantic.

The vast majority of train-related fatalities, over 90 percent, are due to trespassing on railroad tracks, or collisions with road vehicles at level crossings. Organizations such as Operation Lifesaver have been formed to improve safety awareness at railroad crossings, and governments have also launched ad campaigns. Trains cannot stop quickly when at speed; even an emergency brake application may still require more than a mile of stopping distance. As such, emphasis is on educating motorists to yield to trains at crossings and avoid trespassing.

Motive power

Steam locomotives in Zimbabwe

Before steam

The first trains were rope-hauled, gravity powered or pulled by horses.


Steam locomotives work by using a boiler to heat water into steam, which powers the locomotive's pistons which are in turn connected to the wheels. In the mid 20th century, most steam locomotives were replaced by diesel or electric locomotives, which were cheaper, cleaner, and more reliable. Steam locomotives are still used in heritage railways operated in many countries for the leisure and enthusiast market.


Diesel locomotives are powered with a diesel engine, which generates electricity to drive traction motors. This is known as a diesel–electric transmission, and is used on almost all diesels. Diesel power replaced steam for a variety of reasons: diesel locomotives were less complex, far more reliable, cheaper, cleaner, easier to maintain, and more fuel efficient.


Swiss Electric locomotive at Brig, Switzerland

Electric trains receive their current via overhead lines or through a third rail electric system, which is then used to power traction motors that drive the wheels. Electric traction offers a lower cost per mile of train operation but at a higher initial cost, which can only be justified on high traffic lines. Even though the cost per mile of construction is much higher, electric traction is cheaper to operate thanks to lower maintenance and purchase costs for locomotives and equipment. Compared to diesel locomotives, electric locomotives produce no direct emissions and accelerate much faster, making them better suited to passenger service, especially underground.

Other types

Various other types of train propulsion have been tried, some more successful than others.

In the mid 1900s, gas turbine locomotives were developed and successfully used, though most were retired due to high fuel costs and poor reliability.

In the 21st century, alternative fuels for locomotives are under development, due to increasing costs for diesel and a desire to reduce greenhouse gas emissions from trains. Examples include hydrail (trains powered by hydrogen fuel cells) and the use of compressed or liquefied natural gas.

Train cars

Various types of railroad cars in a classification yard in the United States

Train cars, also known as wagons, are unpowered rail vehicles which are typically pulled by locomotives. Many different types exist, specialized to handle various types of cargo. Some common types include boxcars (also known as covered goods wagons) that carry a wide variety of cargo, flatcars (also known as flat wagons) which have flat tops to hold cargo, hopper cars which carry bulk commodities, and tank cars which carry liquids and gases. Examples of more specialized types of train cars include bottle cars which hold molten steel, Schnabel cars which handle very heavy loads, and refrigerator cars which carry perishable goods.

Early train cars were small and light, much like early locomotives, but over time they have become larger as locomotives have become more powerful.

Passenger trains

Second-class compartment of a China Railways CRH1A-A train

A passenger train is used to transport people along a railroad line. These trains may consist of unpowered passenger railroad cars (also known as coaches or carriages) hauled by one or more locomotives, or may be self-propelled; self propelled passenger trains are known as multiple units or railcars. Passenger trains travel between stations or depots, where passengers may board and disembark. In most cases, passenger trains operate on a fixed schedule and have priority over freight trains.

Passenger trains can be divided into short and long distance services.

Long distance trains

Long distance passenger trains travel over hundreds or even thousands of miles between cities. The longest passenger train service in the world is Russia's Trans-Siberian Railway between Moscow and Vladivostok, a distance of 9,289 kilometers (5,772 mi). In general, long distance trains may take days to complete their journeys, and stop at dozens of stations along their routes. For many rural communities, they are the only form of public transportation available.

Short distance trains

Short distance or regional passenger trains have travel times measured in hours or even minutes, as opposed to days. They run more frequently than long distance trains, and are often used by commuters. Short distance passenger trains specifically designed for commuters are known as commuter rail.

High speed trains

The Japanese 0 Series Shinkansen pioneered high speed rail service

High speed trains are designed to be much faster than conventional trains, and typically run on their own separate tracks than other, slower trains. The first high speed train was the Japanese Shinkansen, which opened in 1964. In the 21st century, services such as the French TGV and German Intercity Express are competitive with airplanes in travel time over short to medium distances.

A subset of high speed trains are higher speed trains, which bridge the gap between conventional and high speed trains, and travel at speeds between the two. Examples include the Northeast Regional in the United States, the Gatimaan Express in India, and the KTM ETS in Malaysia.

Rapid transit trains

A number of types of trains are used to provide rapid transit to urban areas. These are distinct from traditional passenger trains in that they operate more frequently, typically do not share tracks with freight trains, and cover relatively short distances. Many different kinds of systems are in use globally.

Rapid transit trains that operate in tunnels below ground are known as subways, undergrounds, or metros. Elevated railways operate on viaducts or bridges above the ground, often on top of city streets. "Metro" may also refer to rapid transit that operates at ground level. In many systems, two or even all three of these types may exist on different portions of a network.

The New Orleans Streetcar System is one of the oldest in the world


Trams, also known in North America as streetcars, typically operate on or parallel to streets in cities, with frequent stops and a high frequency of service.

Light rail

Light rail is a catchall term for a variety of systems, which may include characteristics of trams, passenger trains, and rapid transit systems.

Specialized trains

There are a number of specialized trains which differ from the traditional definition of a train as a set of vehicles which travels on two rails.



Monorails were developed to meet medium-demand traffic in urban transit, and consist of a train running on a single rail, typically elevated. Monorails represent a small proportion of the train systems in use worldwide. Almost all monorail trains use linear induction motors.


To achieve much faster operation over 500 kilometers per hour (310 mph), maglev technology has been researched since the early 20th century. The technology uses magnets to levitate the train above the track, reducing friction and allowing higher speeds. The first commercial maglev train was an airport shuttle introduced in 1984 at Birmingham Airport in England.

The Shanghai Maglev Train, opened in 2003, is the fastest commercial train service of any kind, operating at speeds of up to 430 km/h (270 mph). Japan's L0 Series maglev holds the record for the world's fastest train ever, with a top speed of 603.0 kilometers per hour (374.7 mph). Maglev has not yet been used for inter-city mass transit routes, with only a few examples in use worldwide as of 2021.

Mine trains

Mine trains are operated in large mines and carry both workers and goods. They are usually powered by electricity, to prevent emissions which would pose a health risk to workers underground.

A preserved armored train

Militarized trains

While they have long been important in transporting troops and military equipment, trains have occasionally been used for direct combat. Armored trains have been used in a number of conflicts, as have railroad based artillery systems. Railcar-launched ICBM systems have also been used by nuclear weapon states.

Rack railway

For climbing steep slopes, specialized rack railroads are used. In order to avoid slipping, a rack and pinion system is used, with a toothed rail placed between the two regular rails, which meshes with a drive gear under the locomotive.


Funiculars are also used to climb steep slopes, but instead of a rack use a rope, which is attached to two cars and a pulley. The two funicular cars travel up and down the slope on parallel sets of rails when the pulley is rotated. This design makes funiculars an efficient means of moving people and cargo up and down slopes. The earliest funicular railroad, the Reisszug, opened around 1500.

Freight trains

A Union Pacific freight train; high clearances enable double-stacked containers to be carried in well cars

Freight trains are dedicated to the transport of cargo (also known as goods), rather than people, and are made up of freight cars or wagons. Longer freight trains typically operate between classification yards, while local trains provide freight service between yards and individual loading and unloading points along railroad lines. Major origin or destination points for freight may instead be served by unit trains, which exclusively carry one type of cargo and move directly from the origin to the destination and back without any intermediate stops.

Under the right circumstances, transporting freight by train is less expensive than other modes of transport, and also more energy efficient than transporting freight by road. In the United States, railroads on average moved a ton of freight 702 kilometers (436 mi) per gallon of fuel, as of 2008, an efficiency four times greater than that of trucks. The Environmental and Energy Study Institute estimates that train transportation of freight is between 1.9 and 5.5 times more efficient than by truck, and also generates significantly less pollution. Rail freight is most economic when goods are being carried in bulk and over large distances, but it is less suited to short distances and small loads. With the advent of containerization, freight rail has become part of an intermodal freight network linked with trucking and container ships.

The main disadvantage of rail freight is its lack of flexibility and for this reason, rail has lost much of the freight business to road competition. Many governments are trying to encourage more freight back on to trains because of the community benefits that it would bring.

Cultural impact

Blue toy trains on wooden interlocking tracks on a red rug
A wooden toy train set from Swedish manufacturer Brio

From the dawn of railroading, trains have had a significant cultural impact worldwide. Fast train travel made possible in days or hours journeys which previously took months. Transport of both freight and passengers became far cheaper, allowing for networked economies over large areas. Towns and cities along railroad lines grew in importance, while those bypassed declined or even became ghost towns. Major cities such as Chicago became prominent because they were places where multiple train lines met. In the United States, the completion of the first transcontinental railroad played a major role in the settling of the western part of the nation by non-indigenous migrants and its incorporation into the rest of the country. The Russian Trans-Siberian Railway had a similar impact by connecting the vast country from east to west, and making travel across the frozen Siberia possible.

Trains have long had a major influence on music, art, and literature. Many films heavily involve or are set on trains. Toy train sets are commonly used by children, traditionally boys. Railfans are found around the world, along with hobbyists who create model train layouts. Train enthusiasts generally have a positive relationship with the railroad industry, though sometimes cause issues by trespassing.

Hard disk drive

From Wikipedia, the free encyclopedia

Hard disk drive
IBM 350 RAMAC.jpg
Partially disassembled IBM 350 (RAMAC)
Date inventedDecember 24, 1954; 67 years ago
Invented byIBM team led by Rey Johnson
Internals of a 2.5-inch laptop hard disk drive
A disassembled and labeled 1997 HDD lying atop a mirror

A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data when powered off. Modern HDDs are typically in the form of a small rectangular box.

Introduced by IBM in 1956, HDDs were the dominant secondary storage device for general-purpose computers beginning in the early 1960s. HDDs maintained this position into the modern era of servers and personal computers, though personal computing devices produced in large volume, like cell phones and tablets, rely on flash memory storage devices. More than 224 companies have produced HDDs historically, though after extensive industry consolidation most units are manufactured by Seagate, Toshiba, and Western Digital. HDDs dominate the volume of storage produced (exabytes per year) for servers. Though production is growing slowly (by exabytes shipped), sales revenues and unit shipments are declining because solid-state drives (SSDs) have higher data-transfer rates, higher areal storage density, somewhat better reliability, and much lower latency and access times.

The revenues for SSDs, most of which use NAND flash memory, slightly exceeded those for HDDs in 2018.[14] Flash storage products had more than twice the revenue of hard disk drives as of 2017. Though SSDs have four to nine times higher cost per bit, they are replacing HDDs in applications where speed, power consumption, small size, high capacity and durability are important. As of 2019, the cost per bit of SSDs is falling, and the price premium over HDDs has narrowed.

The primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabytes (GB; where 1 gigabyte = 1 billion (109) bytes). Typically, some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. There can be confusion regarding storage capacity, since capacities are stated in decimal gigabytes (powers of 1000) by HDD manufacturers, whereas the most commonly used operating systems report capacities in powers of 1024, which results in a smaller number than advertised. Performance is specified as the time required to move the heads to a track or cylinder (average access time), the time it takes for the desired sector to move under the head (average latency, which is a function of the physical rotational speed in revolutions per minute), and finally the speed at which the data is transmitted (data rate).

The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops. HDDs are connected to systems by standard interface cables such as PATA (Parallel ATA), SATA (Serial ATA), USB or SAS (Serial Attached SCSI) cables.


Improvement of HDD characteristics over time
Parameter Started with (1957) Improved to Improvement
3.75 megabytes 18 terabytes (as of 2020) 4.8-million-to-one
Physical volume 68 cubic feet (1.9 m3) 2.1 cubic inches (34 cm3) 56,000-to-one
Weight 2,000 pounds
(910 kg)
2.2 ounces
(62 g)
Average access time approx. 600 milliseconds 2.5 ms to 10 ms; RW RAM dependent about
Price US$9,200 per megabyte (1961; US$83,107 in 2021) US$0.024 per gigabyte by 2020 3.46-billion-to-one
Data density 2,000 bits per square inch 1.3 terabits per square inch in 2015 650-million-to-one
Average lifespan c. 2000 hrs MTBF c. 2,500,000 hrs (~285 years) MTBF 1250-to-one

The first production IBM hard disk drive, the 350 disk storage, shipped in 1957 as a component of the IBM 305 RAMAC system. It was approximately the size of two medium-sized refrigerators and stored five million six-bit characters (3.75 megabytes) on a stack of 52 disks (100 surfaces used). The 350 had a single arm with two read/write heads, one facing up and the other down, that moved both horizontally between a pair of adjacent platters and vertically from one pair of platters to a second set. Variants of the IBM 350 were the IBM 355, IBM 7300 and IBM 1405.

In 1961 IBM announced, and in 1962 shipped, the IBM 1301 disk storage unit, which superseded the IBM 350 and similar drives. The 1301 consisted of one (for Model 1) or two (for model 2) modules, each containing 25 platters, each platter about 18-inch (3.2 mm) thick and 24 inches (610 mm) in diameter. While the earlier IBM disk drives used only two read/write heads per arm, the 1301 used an array of 48 heads (comb), each array moving horizontally as a single unit, one head per surface used. Cylinder-mode read/write operations were supported, and the heads flew about 250 micro-inches (about 6 µm) above the platter surface. Motion of the head array depended upon a binary adder system of hydraulic actuators which assured repeatable positioning. The 1301 cabinet was about the size of three home refrigerators placed side by side, storing the equivalent of about 21 million eight-bit bytes per module. Access time was about a quarter of a second.

Also in 1962, IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were called "fixed disk" drives.

In 1963 IBM introduced the 1302, with twice the track capacity and twice as many tracks per cylinder as the 1301. The 1302 had one (for Model 1) or two (for Model 2) modules, each containing a separate comb for the first 250 tracks and the last 250 tracks.

Some high-performance HDDs were manufactured with one head per track, e.g., Burroughs B-475 in 1964, IBM 2305 in 1970, so that no time was lost physically moving the heads to a track and the only latency was the time for the desired block of data to rotate into position under the head. Known as fixed-head or head-per-track disk drives, they were very expensive and are no longer in production.

In 1973, IBM introduced a new type of HDD code-named "Winchester". Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to "land" on a special area of the disk surface upon spin-down, "taking off" again when the disk was later powered on. This greatly reduced the cost of the head actuator mechanism, but precluded removing just the disks from the drive as was done with the disk packs of the day. Instead, the first models of "Winchester technology" drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal. Later "Winchester" drives abandoned the removable media concept and returned to non-removable platters.

In 1974 IBM introduced the swinging arm actuator, made feasible because the Winchester recording heads function well when skewed to the recorded tracks. The simple design of the IBM GV (Gulliver) drive, invented at IBM's UK Hursley Labs, became IBM's most licensed electro-mechanical invention of all time, the actuator and filtration system being adopted in the 1980s eventually for all HDDs, and still universal nearly 40 years and 10 Billion arms later.

Like the first removable pack drive, the first "Winchester" drives used platters 14 inches (360 mm) in diameter. In 1978 IBM introduced a swing arm drive, the IBM 0680 (Piccolo), with eight inch platters, exploring the possibility that smaller platters might offer advantages. Other eight inch drives followed, then 5+14 in (130 mm) drives, sized to replace the contemporary floppy disk drives. The latter were primarily intended for the then fledgling personal computer (PC) market.

Over time, as recording densities were greatly increased, further reductions in disk diameter to 3.5" and 2.5" were found to be optimum. Powerful rare earth magnet materials became affordable during this period, and were complementary to the swing arm actuator design to make possible the compact form factors of modern HDDs.

As the 1980s began, HDDs were a rare and very expensive additional feature in PCs, but by the late 1980s their cost had been reduced to the point where they were standard on all but the cheapest computers.

Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. The subsystem was not sold under the drive manufacturer's name but under the subsystem manufacturer's name such as Corvus Systems and Tallgrass Technologies, or under the PC system manufacturer's name such as the Apple ProFile. The IBM PC/XT in 1983 included an internal 10 MB HDD, and soon thereafter internal HDDs proliferated on personal computers.

External HDDs remained popular for much longer on the Apple Macintosh. Many Macintosh computers made between 1986 and 1998 featured a SCSI port on the back, making external expansion simple. Older compact Macintosh computers did not have user-accessible hard drive bays (indeed, the Macintosh 128K, Macintosh 512K, and Macintosh Plus did not feature a hard drive bay at all), so on those models external SCSI disks were the only reasonable option for expanding upon any internal storage.

HDD improvements have been driven by increasing areal density, listed in the table above. Applications expanded through the 2000s, from the mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content.

In the 2000s and 2010s, NAND began supplanting HDDs in applications requiring portability or high performance. NAND performance is improving faster than HDDs, and applications for HDDs are eroding. In 2018, the largest hard drive had a capacity of 15 TB, while the largest capacity SSD had a capacity of 100 TB. As of 2018, HDDs were forecast to reach 100 TB capacities around 2025, but as of 2019 the expected pace of improvement was pared back to 50 TB by 2026. Smaller form factors, 1.8-inches and below, were discontinued around 2010. The cost of solid-state storage (NAND), represented by Moore's law, is improving faster than HDDs. NAND has a higher price elasticity of demand than HDDs, and this drives market growth. During the late 2000s and 2010s, the product life cycle of HDDs entered a mature phase, and slowing sales may indicate the onset of the declining phase.

The 2011 Thailand floods damaged the manufacturing plants and impacted hard disk drive cost adversely between 2011 and 2013.

In 2019, Western Digital closed its last Malaysian HDD factory due to decreasing demand, to focus on SSD production. All three remaining HDD manufacturers have had decreasing demand for their HDDs since 2014.


Magnetic cross section & frequency modulation encoded binary data

Magnetic recording

A modern HDD records data by magnetizing a thin film of ferromagnetic material on both sides of a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization. User data is encoded using an encoding scheme, such as run-length limited encoding, which determines how the data is represented by the magnetic transitions.

A typical HDD design consists of a spindle that holds flat circular disks, called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminum alloy, glass, or ceramic. They are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection. For reference, a standard piece of copy paper is 0.07–0.18 mm (70,000–180,000 nm) thick.

Destroyed hard disk, glass platter visible
Diagram labeling the major components of a computer HDD
Recording of single magnetisations of bits on a 200 MB HDD-platter (recording made visible using CMOS-MagView).
Longitudinal recording (standard) & perpendicular recording diagram

The platters in contemporary HDDs are spun at speeds varying from 4,200 RPM in energy-efficient portable devices, to 15,000 rpm for high-performance servers. The first HDDs spun at 1,200 rpm and, for many years, 3,600 rpm was the norm. As of November 2019, the platters in most consumer-grade HDDs spin at 5,400 or 7,200 RPM.

Information is written to and read from a platter as it rotates past devices called read-and-write heads that are positioned to operate very close to the magnetic surface, with their flying height often in the range of tens of nanometers. The read-and-write head is used to detect and modify the magnetization of the material passing immediately under it.

In modern drives, there is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or in some older designs a stepper motor. Early hard disk drives wrote data at some constant bits per second, resulting in all tracks having the same amount of data per track but modern drives (since the 1990s) use zone bit recording – increasing the write speed from inner to outer zone and thereby storing more data per track in the outer zones.

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects⁠ ⁠— thermally induced magnetic instability which is commonly known as the "superparamagnetic limit". To counter this, the platters are coated with two parallel magnetic layers, separated by a three-atom layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other. Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, first shipped in 2005, and as of 2007 used in certain HDDs.

In 2004, a higher-density recording media was introduced, consisting of coupled soft and hard magnetic layers. So-called exchange spring media magnetic storage technology, also known as exchange coupled composite media, allows good writability due to the write-assist nature of the soft layer. However, the thermal stability is determined only by the hardest layer and not influenced by the soft layer.


An HDD with disks and motor hub removed, exposing copper-colored stator coils surrounding a bearing in the center of the spindle motor. The orange stripe along the side of the arm is a thin printed-circuit cable, the spindle bearing is in the center and the actuator is in the upper left.

A typical HDD has two electric motors: a spindle motor that spins the disks and an actuator (motor) that positions the read/write head assembly across the spinning disks. The disk motor has an external rotor attached to the disks; the stator windings are fixed in place. Opposite the actuator at the end of the head support arm is the read-write head; thin printed-circuit cables connect the read-write heads to amplifier electronics mounted at the pivot of the actuator. The head support arm is very light, but also stiff; in modern drives, acceleration at the head reaches 550 g.

Head stack with an actuator coil on the left and read/write heads on the right
Close-up of a single read-write head, showing the side facing the platter

The actuator is a permanent magnet and moving coil motor that swings the heads to the desired position. A metal plate supports a squat neodymium-iron-boron (NIB) high-flux magnet. Beneath this plate is the moving coil, often referred to as the voice coil by analogy to the coil in loudspeakers, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor (some drives have only one magnet).

The voice coil itself is shaped rather like an arrowhead and is made of doubly coated copper magnet wire. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it is wound on a form, making it self-supporting. The portions of the coil along the two sides of the arrowhead (which point to the center of the actuator bearing) then interact with the magnetic field of the fixed magnet. Current flowing radially outward along one side of the arrowhead and radially inward on the other produces the tangential force. If the magnetic field were uniform, each side would generate opposing forces that would cancel each other out. Therefore, the surface of the magnet is half north pole and half south pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling. Currents along the top and bottom of the coil produce radial forces that do not rotate the head.

The HDD's electronics control the movement of the actuator and the rotation of the disk and perform reads and writes on demand from the disk controller. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback. These are either complete concentric circles (in the case of dedicated servo technology) or segments interspersed with real data (in the case of embedded servo technology). The servo feedback optimizes the signal-to-noise ratio of the GMR sensors by adjusting the voice coil of the actuated arm. The spinning of the disk also uses a servo motor. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media that have failed.

Error rates and handling

Modern drives make extensive use of error correction codes (ECCs), particularly Reed–Solomon error correction. These techniques store extra bits, determined by mathematical formulas, for each block of data; the extra bits allow many errors to be corrected invisibly. The extra bits themselves take up space on the HDD, but allow higher recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity. For example, a typical 1 TB hard disk with 512-byte sectors provides additional capacity of about 93 GB for the ECC data.

In the newest drives, as of 2009, low-density parity-check codes (LDPC) were supplanting Reed–Solomon; LDPC codes enable performance close to the Shannon Limit and thus provide the highest storage density available.

Typical hard disk drives attempt to "remap" the data in a physical sector that is failing to a spare physical sector provided by the drive's "spare sector pool" (also called "reserve pool"), while relying on the ECC to recover stored data while the number of errors in a bad sector is still low enough. The S.M.A.R.T (Self-Monitoring, Analysis and Reporting Technology) feature counts the total number of errors in the entire HDD fixed by ECC (although not on all hard drives as the related S.M.A.R.T attributes "Hardware ECC Recovered" and "Soft ECC Correction" are not consistently supported), and the total number of performed sector remappings, as the occurrence of many such errors may predict an HDD failure.

The "No-ID Format", developed by IBM in the mid-1990s, contains information about which sectors are bad and where remapped sectors have been located.

Only a tiny fraction of the detected errors end up as not correctable. Examples of specified uncorrected bit read error rates include:

  • 2013 specifications for enterprise SAS disk drives state the error rate to be one uncorrected bit read error in every 1016 bits read,
  • 2018 specifications for consumer SATA hard drives state the error rate to be one uncorrected bit read error in every 1014 bits.

Within a given manufacturers model the uncorrected bit error rate is typically the same regardless of capacity of the drive.

The worst type of errors are silent data corruptions which are errors undetected by the disk firmware or the host operating system; some of these errors may be caused by hard disk drive malfunctions while others originate elsewhere in the connection between the drive and the host.


Leading-edge hard disk drive areal densities from 1956 through 2009 compared to Moore's law. By 2016, progress had slowed significantly below the extrapolated density trend.

The rate of areal density advancement was similar to Moore's law (doubling every two years) through 2010: 60% per year during 1988–1996, 100% during 1996–2003 and 30% during 2003–2010. Speaking in 1997, Gordon Moore called the increase "flabbergasting", while observing later that growth cannot continue forever. Price improvement decelerated to −12% per year during 2010–2017, as the growth of areal density slowed. The rate of advancement for areal density slowed to 10% per year during 2010–2016, and there was difficulty in migrating from perpendicular recording to newer technologies.

As bit cell size decreases, more data can be put onto a single drive platter. In 2013, a production desktop 3 TB HDD (with four platters) would have had an areal density of about 500 Gbit/in2 which would have amounted to a bit cell comprising about 18 magnetic grains (11 by 1.6 grains). Since the mid-2000s areal density progress has been challenged by a superparamagnetic trilemma involving grain size, grain magnetic strength and ability of the head to write. In order to maintain acceptable signal to noise smaller grains are required; smaller grains may self-reverse (electrothermal instability) unless their magnetic strength is increased, but known write head materials are unable to generate a strong enough magnetic field sufficient to write the medium in the increasingly smaller space taken by grains.

Magnetic storage technologies are being developed to address this trilemma, and compete with flash memory–based solid-state drives (SSDs). In 2013, Seagate introduced shingled magnetic recording (SMR), intended as something of a "stopgap" technology between PMR and Seagate's intended successor heat-assisted magnetic recording (HAMR), SMR utilises overlapping tracks for increased data density, at the cost of design complexity and lower data access speeds (particularly write speeds and random access 4k speeds).

By contrast, HGST (now part of Western Digital) focused on developing ways to seal helium-filled drives instead of the usual filtered air. Since turbulence and friction are reduced, higher areal densities can be achieved due to using a smaller track width, and the energy dissipated due to friction is lower as well, resulting in a lower power draw. Furthermore, more platters can be fit into the same enclosure space, although helium gas is notoriously difficult to prevent escaping. Thus, helium drives are completely sealed and do not have a breather port, unlike their air-filled counterparts.

Other recording technologies are either under research or have been commercially implemented to increase areal density, including Seagate's heat-assisted magnetic recording (HAMR). HAMR requires a different architecture with redesigned media and read/write heads, new lasers, and new near-field optical transducers. HAMR is expected to ship commercially in late 2020 or 2021. Technical issues delayed the introduction of HAMR by a decade, from earlier projections of 2009, 2015, 2016, and the first half of 2019. Some drives have adopted dual independent actuator arms to increase read/write speeds and compete with SSDs. HAMR's planned successor, bit-patterned recording (BPR), has been removed from the roadmaps of Western Digital and Seagate. Western Digital's microwave-assisted magnetic recording (MAMR), also referred to as energy-assisted magnetic recording (EAMR), was sampled in 2020, with the first EAMR drive, the Ultrastar HC550, shipping in late 2020. Two-dimensional magnetic recording (TDMR) and "current perpendicular to plane" giant magnetoresistance (CPP/GMR) heads have appeared in research papers. A 3D-actuated vacuum drive (3DHD) concept has been proposed.

The rate of areal density growth had dropped below the historical Moore's law rate of 40% per year by 2016. Depending upon assumptions on feasibility and timing of these technologies, Seagate forecasts that areal density will grow 20% per year during 2020–2034.


Two Seagate Barracuda drives, from 2003 and 2009 - respectively 160GB and 1TB. As of 2022 Seagate offers capacities up to 20TB.

The highest-capacity HDDs shipping commercially in 2022 are 20 TB.

The capacity of a hard disk drive, as reported by an operating system to the end user, is smaller than the amount stated by the manufacturer for several reasons, e.g., the operating system using some space, use of some space for data redundancy, space use for file system structures. Also the difference in capacity reported in SI decimal prefixed units vs. binary prefixes can lead to a false impression of missing capacity.


Modern hard disk drives appear to their host controller as a contiguous set of logical blocks, and the gross drive capacity is calculated by multiplying the number of blocks by the block size. This information is available from the manufacturer's product specification, and from the drive itself through use of operating system functions that invoke low-level drive commands.

Older IBM and compatible drives, e.g., IBM 3390, using the CKD record format have variable length records; such drive capacity calculations must take into account the characteristics of the records. Some newer DASD simulate CKD, and the same capacity formulae apply.

The gross capacity of older sector-oriented HDDs is calculated as the product of the number of cylinders per recording zone, the number of bytes per sector (most commonly 512), and the count of zones of the drive. Some modern SATA drives also report cylinder-head-sector (CHS) capacities, but these are not physical parameters because the reported values are constrained by historic operating system interfaces. The C/H/S scheme has been replaced by logical block addressing (LBA), a simple linear addressing scheme that locates blocks by an integer index, which starts at LBA 0 for the first block and increments thereafter. When using the C/H/S method to describe modern large drives, the number of heads is often set to 64, although a typical modern hard disk drive has between one and four platters. In modern HDDs, spare capacity for defect management is not included in the published capacity; however, in many early HDDs a certain number of sectors were reserved as spares, thereby reducing the capacity available to the operating system. Furthermore, many HDDs store their firmware in a reserved service zone, which is typically not accessible by the user, and is not included in the capacity calculation.

For RAID subsystems, data integrity and fault-tolerance requirements also reduce the realized capacity. For example, a RAID 1 array has about half the total capacity as a result of data mirroring, while a RAID 5 array with n drives loses 1/n of capacity (which equals to the capacity of a single drive) due to storing parity information. RAID subsystems are multiple drives that appear to be one drive or more drives to the user, but provide fault tolerance. Most RAID vendors use checksums to improve data integrity at the block level. Some vendors design systems using HDDs with sectors of 520 bytes to contain 512 bytes of user data and eight checksum bytes, or by using separate 512-byte sectors for the checksum data.

Some systems may use hidden partitions for system recovery, reducing the capacity available to the end user without knowledge of special disk partitioning utilities like diskpart in Windows.


Data is stored on a hard drive in a series of logical blocks. Each block is delimited by markers identifying its start and end, error detecting and correcting information, and space between blocks to allow for minor timing variations. These blocks often contained 512 bytes of usable data, but other sizes have been used. As drive density increased, an initiative known as Advanced Format extended the block size to 4096 bytes of usable data, with a resulting significant reduction in the amount of disk space used for block headers, error checking data, and spacing.

The process of initializing these logical blocks on the physical disk platters is called low-level formatting, which is usually performed at the factory and is not normally changed in the field. High-level formatting writes data structures used by the operating system to organize data files on the disk. This includes writing partition and file system structures into selected logical blocks. For example, some of the disk space will be used to hold a directory of disk file names and a list of logical blocks associated with a particular file.

Examples of partition mapping scheme include Master boot record (MBR) and GUID Partition Table (GPT). Examples of data structures stored on disk to retrieve files include the File Allocation Table (FAT) in the DOS file system and inodes in many UNIX file systems, as well as other operating system data structures (also known as metadata). As a consequence, not all the space on an HDD is available for user files, but this system overhead is usually small compared with user data.


Decimal and binary unit prefixes interpretation
Capacity advertised by manufacturers Capacity expected by some consumers Reported capacity
Windows macOS ver 10.6+
With prefix Bytes Bytes Diff.
100 GB 100,000,000,000 107,374,182,400 7.37% 93.1 GB 100 GB
TB 1,000,000,000,000 1,099,511,627,776 9.95% 931 GB 1,000 GB, 1,000,000 MB

In the early days of computing the total capacity of HDDs was specified in 7 to 9 decimal digits frequently truncated with the idiom millions. By the 1970s, the total capacity of HDDs was given by manufacturers using SI decimal prefixes such as megabytes (1 MB = 1,000,000 bytes), gigabytes (1 GB = 1,000,000,000 bytes) and terabytes (1 TB = 1,000,000,000,000 bytes). However, capacities of memory are usually quoted using a binary interpretation of the prefixes, i.e. using powers of 1024 instead of 1000.

Software reports hard disk drive or memory capacity in different forms using either decimal or binary prefixes. The Microsoft Windows family of operating systems uses the binary convention when reporting storage capacity, so an HDD offered by its manufacturer as a 1 TB drive is reported by these operating systems as a 931 GB HDD. Mac OS X 10.6 ("Snow Leopard") uses decimal convention when reporting HDD capacity. The default behavior of the df command-line utility on Linux is to report the HDD capacity as a number of 1024-byte units.

The difference between the decimal and binary prefix interpretation caused some consumer confusion and led to class action suits against HDD manufacturers. The plaintiffs argued that the use of decimal prefixes effectively misled consumers while the defendants denied any wrongdoing or liability, asserting that their marketing and advertising complied in all respects with the law and that no class member sustained any damages or injuries.

Price evolution

HDD price per byte decreased at the rate of 40% per year during 1988–1996, 51% per year during 1996–2003 and 34% per year during 2003–2010. The price decrease slowed down to 13% per year during 2011–2014, as areal density increase slowed and the 2011 Thailand floods damaged manufacturing facilities and have held at 11% per year during 2010–2017.

The Federal Reserve Board has published a quality-adjusted price index for large-scale enterprise storage systems including three or more enterprise HDDs and associated controllers, racks and cables. Prices for these large-scale storage systems decreased at the rate of 30% per year during 2004–2009 and 22% per year during 2009–2014.

Form factors

8-, 5.25-, 3.5-, 2.5-, 1.8- and 1-inch HDDs, together with a ruler to show the size of platters and read-write heads
A newer 2.5-inch (63.5 mm) 6,495 MB HDD compared to an older 5.25-inch full-height 110 MB HDD

IBM's first hard disk drive, the IBM 350, used a stack of fifty 24-inch platters, stored 3.75 MB of data (approximately the size of one modern digital picture), and was of a size comparable to two large refrigerators. In 1962, IBM introduced its model 1311 disk, which used six 14-inch (nominal size) platters in a removable pack and was roughly the size of a washing machine. This became a standard platter size for many years, used also by other manufacturers. The IBM 2314 used platters of the same size in an eleven-high pack and introduced the "drive in a drawer" layout. sometimes called the"pizza oven", although the "drawer" was not the complete drive. Into the 1970s HDDs were offered in standalone cabinets of varying dimensions containing from one to four HDDs.

Beginning in the late 1960s drives were offered that fit entirely into a chassis that would mount in a 19-inch rack. Digital's RK05 and RL01 were early examples using single 14-inch platters in removable packs, the entire drive fitting in a 10.5-inch-high rack space (six rack units). In the mid-to-late 1980s the similarly sized Fujitsu Eagle, which used (coincidentally) 10.5-inch platters, was a popular product.

With increasing sales of microcomputers having built in floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable. Starting with the Shugart Associates SA1000, HDD form factors initially followed those of 8-inch, 5¼-inch, and 3½-inch floppy disk drives. Although referred to by these nominal sizes, the actual sizes for those three drives respectively are 9.5", 5.75" and 4" wide. Because there were no smaller floppy disk drives, smaller HDD form factors such as 2½-inch drives (actually 2.75" wide) developed from product offerings or industry standards.

As of 2019, 2½-inch and 3½-inch hard disks are the most popular sizes. By 2009, all manufacturers had discontinued the development of new products for the 1.3-inch, 1-inch and 0.85-inch form factors due to falling prices of flash memory, which has no moving parts. While nominal sizes are in inches, actual dimensions are specified in millimeters.

Performance characteristics

The factors that limit the time to access the data on an HDD are mostly related to the mechanical nature of the rotating disks and moving heads, including:

  • Seek time is a measure of how long it takes the head assembly to travel to the track of the disk that contains data.
  • Rotational latency is incurred because the desired disk sector may not be directly under the head when data transfer is requested. Average rotational latency is shown in the table, based on the statistical relation that the average latency is one-half the rotational period.
  • The bit rate or data transfer rate (once the head is in the right position) creates delay which is a function of the number of blocks transferred; typically relatively small, but can be quite long with the transfer of large contiguous files.

Delay may also occur if the drive disks are stopped to save energy.

Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk. Some computer operating systems perform defragmentation automatically. Although automatic defragmentation is intended to reduce access delays, performance will be temporarily reduced while the procedure is in progress.

Time to access data can be improved by increasing rotational speed (thus reducing latency) or by reducing the time spent seeking. Increasing areal density increases throughput by increasing data rate and by increasing the amount of data under a set of heads, thereby potentially reducing seek activity for a given amount of data. The time to access data has not kept up with throughput increases, which themselves have not kept up with growth in bit density and storage capacity.


Latency characteristics typical of HDDs
Rotational speed
Average rotational latency
15,000 2
10,000 3
7,200 4.16
5,400 5.55
4,800 6.25

Data transfer rate

As of 2010, a typical 7,200-rpm desktop HDD has a sustained "disk-to-buffer" data transfer rate up to 1,030 Mbit/s. This rate depends on the track location; the rate is higher for data on the outer tracks (where there are more data sectors per rotation) and lower toward the inner tracks (where there are fewer data sectors per rotation); and is generally somewhat higher for 10,000-rpm drives. A current widely used standard for the "buffer-to-computer" interface is 3.0 Gbit/s SATA, which can send about 300 megabyte/s (10-bit encoding) from the buffer to the computer, and thus is still comfortably ahead of today's disk-to-buffer transfer rates. Data transfer rate (read/write) can be measured by writing a large file to disk using special file generator tools, then reading back the file. Transfer rate can be influenced by file system fragmentation and the layout of the files.

HDD data transfer rate depends upon the rotational speed of the platters and the data recording density. Because heat and vibration limit rotational speed, advancing density becomes the main method to improve sequential transfer rates. Higher speeds require a more powerful spindle motor, which creates more heat. While areal density advances by increasing both the number of tracks across the disk and the number of sectors per track, only the latter increases the data transfer rate for a given rpm. Since data transfer rate performance tracks only one of the two components of areal density, its performance improves at a lower rate.

Other considerations

Other performance considerations include quality-adjusted price, power consumption, audible noise, and both operating and non-operating shock resistance.

Access and interfaces

Inner view of a 1998 Seagate HDD that used the Parallel ATA interface
2.5-inch SATA drive on top of 3.5-inch SATA drive, showing close-up of (7-pin) data and (15-pin) power connectors

Current hard drives connect to a computer over one of several bus types, including parallel ATA, Serial ATA, SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Some drives, especially external portable drives, use IEEE 1394, or USB. All of these interfaces are digital; electronics on the drive process the analog signals from the read/write heads. Current drives present a consistent interface to the rest of the computer, independent of the data encoding scheme used internally, and independent of the physical number of disks and heads within the drive.

Typically a DSP in the electronics inside the drive takes the raw analog voltages from the read head and uses PRML and Reed–Solomon error correction to decode the data, then sends that data out the standard interface. That DSP also watches the error rate detected by error detection and correction, and performs bad sector remapping, data collection for Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks.

Modern interfaces connect the drive to the host interface with a single data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit. Older interfaces had separate cables for data signals and for drive control signals.

  • Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, was standard on servers, workstations, Commodore Amiga, Atari ST and Apple Macintosh computers through the mid-1990s, by which time most models had been transitioned to newer interfaces. The length limit of the data cable allows for external SCSI devices. The SCSI command set is still used in the more modern SAS interface.
  • Integrated Drive Electronics (IDE), later standardized under the name AT Attachment (ATA, with the alias PATA (Parallel ATA) retroactively added upon introduction of SATA) moved the HDD controller from the interface card to the disk drive. This helped to standardize the host/controller interface, reduce the programming complexity in the host device driver, and reduced system cost and complexity. The 40-pin IDE/ATA connection transfers 16 bits of data at a time on the data cable. The data cable was originally 40-conductor, but later higher speed requirements led to an "ultra DMA" (UDMA) mode using an 80-conductor cable with additional wires to reduce crosstalk at high speed.
  • EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA eliminates the need for the CPU to copy byte per byte, therefore allowing it to process other tasks while the data transfer occurs.
  • Fibre Channel (FC) is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, and it is the cornerstone of storage area networks (SANs). Recently other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly, drives usually use copper twisted-pair cables for Fibre Channel, not fibre optics. The latter are traditionally reserved for larger devices, such as servers or disk array controllers.
  • Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses a mechanically compatible data and power connector to standard 3.5-inch SATA1/SATA2 HDDs, and many server-oriented SAS RAID controllers are also capable of addressing SATA HDDs. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.
  • Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. A similar differential signaling system is used in RS485, LocalTalk, USB, FireWire, and differential SCSI. SATA I to III are designed to be compatible with, and use, a subset of SAS commands, and compatible interfaces. Therefore, a SATA hard drive can be connected to and controlled by a SAS hard drive controller (with some minor exceptions such as drives/controllers with limited compatibility). However they cannot be connected the other way round—a SATA controller cannot be connected to a SAS drive.

Integrity and failure

Close-up of an HDD head resting on a disk platter; its mirror reflection is visible on the platter surface. Unless the head is on a landing zone, the heads touching the platters while in operation can be catastrophic.

Due to the extremely close spacing between the heads and the disk surface, HDDs are vulnerable to being damaged by a head crash – a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, contamination of the drive's internal enclosure, wear and tear, corrosion, or poorly manufactured platters and heads.

The HDD's spindle system relies on air density inside the disk enclosure to support the heads at their proper flying height while the disk rotates. HDDs require a certain range of air densities to operate properly. The connection to the external environment and density occurs through a small hole in the enclosure (about 0.5 mm in breadth), usually with a filter on the inside (the breather filter). If the air density is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 3,000 m (9,800 ft). Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on all disk drives – they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating drive is constantly moving too, being swept in motion by friction with the spinning platters. This air passes through an internal recirculation (or "recirc") filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the enclosure, and any particles or outgassing generated internally in normal operation. Very high humidity present for extended periods of time can corrode the heads and platters. An exception to this are hermetically sealed, helium filled HDDs that largely eliminate environmental issues that can arise due to humidity or atmospheric pressure changes. Such HDDs were introduced by HGST in their first successful high volume implementation in 2013.

For giant magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes (so called "thermal asperity", a problem which can partially be dealt with by proper electronic filtering of the read signal).

When the logic board of a hard disk fails, the drive can often be restored to functioning order and the data recovered by replacing the circuit board with one of an identical hard disk. In the case of read-write head faults, they can be replaced using specialized tools in a dust-free environment. If the disk platters are undamaged, they can be transferred into an identical enclosure and the data can be copied or cloned onto a new drive. In the event of disk-platter failures, disassembly and imaging of the disk platters may be required. For logical damage to file systems, a variety of tools, including fsck on UNIX-like systems and CHKDSK on Windows, can be used for data recovery. Recovery from logical damage can require file carving.

A common expectation is that hard disk drives designed and marketed for server use will fail less frequently than consumer-grade drives usually used in desktop computers. However, two independent studies by Carnegie Mellon University and Google found that the "grade" of a drive does not relate to the drive's failure rate.

A 2011 summary of research, into SSD and magnetic disk failure patterns by Tom's Hardware summarized research findings as follows:

  • Mean time between failures (MTBF) does not indicate reliability; the annualized failure rate is higher and usually more relevant.
  • HDDs do not tend to fail during early use, and temperature has only a minor effect; instead, failure rates steadily increase with age.
  • S.M.A.R.T. warns of mechanical issues but not other issues affecting reliability, and is therefore not a reliable indicator of condition.
  • Failure rates of drives sold as "enterprise" and "consumer" are "very much similar", although these drive types are customized for their different operating environments.
  • In drive arrays, one drive's failure significantly increases the short-term risk of a second drive failing.

As of 2019, Backblaze, a storage provider reported an annualized failure rate of two percent per year for a storage farm with 110,000 off-the-shelf HDDs with the reliability varying widely between models and manufacturers. Backblaze subsequently reported that the failure rate for HDDs and SSD of equivalent age was similar.

To minimize cost and overcome failures of individual HDDs, storage systems providers rely on redundant HDD arrays. HDDs that fail are replaced on an ongoing basis.

Market segments

Consumer segment

Two high-end consumer SATA 2.5-inch 10,000 rpm HDDs, factory-mounted in 3.5-inch adapter frames
Desktop HDDs
Desktop HDDs typically have two to five internal platters, rotate at 5,400 to 10,000 rpm, and have a media transfer rate of 0.5 Gbit/s or higher (1 GB = 109 bytes; 1 Gbit/s = 109 bit/s). Earlier (1980-1990s) drives tend to be slower in rotation speed. As of May 2019, the highest-capacity desktop HDDs stored 16 TB, with plans to release 18 TB drives later in 2019. 18 TB HDDs were released in 2020. As of 2016, the typical speed of a hard drive in an average desktop computer is 7,200 RPM, whereas low-cost desktop computers may use 5,900 RPM or 5,400 RPM drives. For some time in the 2000s and early 2010s some desktop users and data centers also used 10,000 RPM drives such as Western Digital Raptor but such drives have become much rarer as of 2016 and are not commonly used now, having been replaced by NAND flash-based SSDs.
Mobile (laptop) HDDs
Smaller than their desktop and enterprise counterparts, they tend to be slower and have lower capacity, because typically has one internal platter and were 2.5" or 1.8" physical size instead of more common for desktops 3.5" form-factor. Mobile HDDs spin at 4,200 rpm, 5,200 rpm, 5,400 rpm, or 7,200 rpm, with 5,400 rpm being the most common. 7,200 rpm drives tend to be more expensive and have smaller capacities, while 4,200 rpm models usually have very high storage capacities. Because of smaller platter(s), mobile HDDs generally have lower capacity than their desktop counterparts.
Consumer electronics HDDs
They include drives embedded into digital video recorders and automotive vehicles. The former are configured to provide a guaranteed streaming capacity, even in the face of read and write errors, while the latter are built to resist larger amounts of shock. They usually spin at a speed of 5400 RPM.
External and portable HDDs
Two 2.5" external USB hard drives
Current external hard disk drives typically connect via USB-C; earlier models use an regular USB (sometimes with using of a pair of ports for better bandwidth) or (rarely), e.g., eSATA connection. Variants using USB 2.0 interface generally have slower data transfer rates when compared to internally mounted hard drives connected through SATA. Plug and play drive functionality offers system compatibility and features large storage options and portable design. As of March 2015, available capacities for external hard disk drives ranged from 500 GB to 10 TB. External hard disk drives are usually available as assembled integrated products but may be also assembled by combining an external enclosure (with USB or other interface) with a separately purchased drive. They are available in 2.5-inch and 3.5-inch sizes; 2.5-inch variants are typically called portable external drives, while 3.5-inch variants are referred to as desktop external drives. "Portable" drives are packaged in smaller and lighter enclosures than the "desktop" drives; additionally, "portable" drives use power provided by the USB connection, while "desktop" drives require external power bricks. Features such as encryption, Wi-Fi connectivity, biometric security or multiple interfaces (for example, FireWire) are available at a higher cost. There are pre-assembled external hard disk drives that, when taken out from their enclosures, cannot be used internally in a laptop or desktop computer due to embedded USB interface on their printed circuit boards, and lack of SATA (or Parallel ATA) interfaces.

Enterprise and business segment

Server and workstation HDDs
Hot-swappable HDD enclosure
Typically used with multiple-user computers running enterprise software. Examples are: transaction processing databases, internet infrastructure (email, webserver, e-commerce), scientific computing software, and nearline storage management software. Enterprise drives commonly operate continuously ("24/7") in demanding environments while delivering the highest possible performance without sacrificing reliability. Maximum capacity is not the primary goal, and as a result the drives are often offered in capacities that are relatively low in relation to their cost.
The fastest enterprise HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s and a sustained transfer rate up to 1 Gbit/s. Drives running at 10,000 or 15,000 rpm use smaller platters to mitigate increased power requirements (as they have less air drag) and therefore generally have lower capacity than the highest capacity desktop drives. Enterprise HDDs are commonly connected through Serial Attached SCSI (SAS) or Fibre Channel (FC). Some support multiple ports, so they can be connected to a redundant host bus adapter.
Enterprise HDDs can have sector sizes larger than 512 bytes (often 520, 524, 528 or 536 bytes). The additional per-sector space can be used by hardware RAID controllers or applications for storing Data Integrity Field (DIF) or Data Integrity Extensions (DIX) data, resulting in higher reliability and prevention of silent data corruption.
Video recording HDDs
This line were similar to consumer video recording HDDs with stream stability requirements and similar to server HDDs with requirements to expandability support, but also they strongly oriented for growing of internal capacity. The main sacrifice for this segment is a writing and reading speed.

Manufacturers and sales

Diagram of HDD manufacturer consolidation

More than 200 companies have manufactured HDDs over time, but consolidations have concentrated production to just three manufacturers today: Western Digital, Seagate, and Toshiba. Production is mainly in the Pacific rim.

Worldwide revenue for disk storage declined eight percent per year, from a peak of $38 billion in 2012 to $22 billion (estimated) in 2019. Production of HDD storage grew 15% per year during 2011–2017, from 335 to 780 exabytes per year. HDD shipments declined seven percent per year during this time period, from 620 to 406 million units. HDD shipments were projected to drop by 18% during 2018–2019, from 375 million to 309 million units. In 2018, Seagate has 40% of unit shipments, Western Digital has 37% of unit shipments, while Toshiba has 23% of unit shipments. The average sales price for the two largest manufacturers was $60 per unit in 2015.

Competition from SSDs

HDDs are being superseded by solid-state drives (SSDs) in markets where their higher speed (up to 4950 megabytes) (4.95 gigabytes) per second for M.2 (NGFF) NVMe SSDs, or 2500 megabytes (2.5 gigabytes) per second for PCIe expansion card drives), ruggedness, and lower power are more important than price, since the bit cost of SSDs is four to nine times higher than HDDs. As of 2016, HDDs are reported to have a failure rate of 2–9% per year, while SSDs have fewer failures: 1–3% per year. However, SSDs have more un-correctable data errors than HDDs.

SSDs offer larger capacities (up to 100 TB) than the largest HDD and/or higher storage densities (100 TB and 30 TB SSDs are housed in 2.5 inch HDD cases but with the same height as a 3.5-inch HDD), although their cost remains prohibitive.

A laboratory demonstration of a 1.33-Tb 3D NAND chip with 96 layers (NAND commonly used in solid state drives (SSDs)) had 5.5 Tbit/in2 as of 2019, while the maximum areal density for HDDs is 1.5 Tbit/in2. The areal density of flash memory is doubling every two years, similar to Moore's law (40% per year) and faster than the 10–20% per year for HDDs. As of 2018, the maximum capacity was 16 terabytes for an HDD, and 100 terabytes for an SSD. HDDs were used in 70% of the desktop and notebook computers produced in 2016, and SSDs were used in 30%. The usage share of HDDs is declining and could drop below 50% in 2018–2019 according to one forecast, because SSDs are replacing smaller-capacity (less than one-terabyte) HDDs in desktop and notebook computers and MP3 players.

The market for silicon-based flash memory (NAND) chips, used in SSDs and other applications, is growing faster than for HDDs. Worldwide NAND revenue grew 16% per year from $22 billion to $57 billion during 2011–2017, while production grew 45% per year from 19 exabytes to 175 exabytes.


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