hard disk

A hard disk is part of a unit -- often called a disk drive, hard drive or hard disk drive -- that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. Today's computers typically come with a hard disk that can contain anywhere from billions to trillions of bytes of storage.

A hard disk is actually a set of stacked disks, like phonograph records. Each disk has data recorded electromagnetically in concentric circles, or tracks, on the disk. A head, similar to a phonograph arm but in a relatively fixed position, writes or reads the information on the tracks. Two heads, one on each side of a disk, read or write the data as the disk spins. Each read or write operation requires that data be located, an operation called a seek. Data already in a disk cache, however, will be located more quickly.

A hard disk/drive unit comes with a set rotation speed varying from 4,200 revolutions per minute to 15,000 rpm. Most laptop and desktop PCs use hard disks that fall between 5,400 rpm and 7,200 rpm, while hard disks at higher rpm can be found in high-end workstations and enterprise servers. Disk access time is measured in milliseconds. Although the physical location of data can be identified with cylinder, track and sector locations, these are actually mapped to a logical block address (LBA) that works with the larger address range on hard disks.

Hard disks remain a popular data storage option for consumers and enterprises, in spite of the growing popularity and rapidly lowering cost of nonvolatile solid-state flash memory in the form of solid-state drives (SSDs). SSDs fit into the same external and internal drive bays as their HDD counterparts. SSDs may be much faster, more durable and draw less power than hard disks, but they are also more expensive. SSDs are considered a better fit for applications that demand high performance, while HDDs are more often used for high-capacity use cases.


In 1953, IBM engineers created the first hard disk, which was the size of two refrigerators. The company then shipped the first commercial hard disk-based computer, the 5 MB IBM 305 RAMAC (random access method of accounting and control) in 1956. The storage component of the IBM 305 RAMAC was called IBM 350 Disk Storage. RAMAC disks were 2 feet in diameter, and the storage cost worked out to approximately $10,000 per megabyte. It was nevertheless a huge jump forward in computer storage technology, which had mostly been reliant on magnetic tape. The movable read and write heads of RAMAC enabled semirandom access to data for the first time.

IBM continued to lead the development of hard disk technology over the next couple of decades. In 1961, the drive heads of the IBM 1301 Disk Storage Unit floated on a thin layer of air, which kept the heads and platters closer for an increase in storage density. A couple of years later, IBM introduced the first removable hard drive, the 1311. Its first disk pack, the IBM 1316, consisted of six 14-inch platters and 2.6 MB of storage. This was followed by the IBM 2311 (5 MB) and IBM 2314 (29 MB) disk pack HDDs, the latter the first to be standardized, as it worked across multiple editions of the IBM System/360 mainframe computer system.

Memorex introduced the first IBM-compatible hard disk in 1968. In 1970, the first hard drives with error correction appeared, and Western Digital (at the time named General Digital Corporation) was established. In 1973, IBM released the 3340 -- known as the "Winchester" -- the first sealed hard drive with low-mass heads and lubricated spindles. The first patent for redundant array of independent disks (RAID) technology was filed in 1978, and in 1979, a group headed by Al Shugart, who helped develop RAMAC decades earlier, founded Seagate Technology Corp. That was also the year IBM rolled out its Piccolo drive, which used eight disks to store 64 MB, and the IBM 3370, the first HDD with a thin-film head.

In 1980, IBM released the first gigabyte hard drive, which weighed 550 pounds and was the size of a refrigerator, for $40,000. This was the same year Seagate introduced the first 5.25-inch hard disk. Scottish company Rodime produced a 3.5-inch hard drive in 1983. Three years later, the Small Computer System Interface (SCSI) standard came along. In 1988, PrairieTek shrank the hard drive to 2.5 inches -- 20 MB on two disk platters -- for use in laptops.

With the dawn of the 1990s came IBM's 0663 Corsair drive. Storing up to 1 GB of data on 8.95 mm disks, the 0663 was the first hard disk to use magnetoresistive heads. Drives continued to shrink, with the first 1.8-inch disk coming from Integra Peripherals in 1991 followed by the 1.3-inch Hewlett Packard Kittyhawk in 1992. Western Digital developed the Enhanced IDE hard drive interface, breaking the 528 MB throughput barrier, in 1994. By 1996, IBM was storing 1 billion bits per square inch on a platter, and Seagate's Cheetah family became the first 10,000 rpm HDDs. In 1997, IBM rolled out the 3.5-inch 16.8 GB Titan, the first hard disk to use giant magnetoresistive heads. The company's Microdrive, released the next year, stored 340 MB on a single 1-inch disk platter.

In 2000, Maxtor bought competitor Quantum's hard drive business, and Seagate hit 15,000 rpm with its Cheetah X15 HDD. Seagate achieved another milestone by demonstrating a perpendicular magnetic recording areal density of 100 Gb per square inch that same year. Hitachi bought IBM's data storage business in 2003, Seagate produced the first Serial Advanced Technology Attachment computer bus interface and Western Digital made the first 10,000 rpm SATA hard drive, the 37 GB Raptor. In 2004, Toshiba released the first 0.85-inch hard drive, a 2 GB model on a single platter, while Hitachi shipped the first 500 GB HDD in 2005. In 2006, Seagate acquired Maxtor, further consolidating the hard drive market. Drive capacities continued to skyrocket from there.


By the end of the 2000s, Seagate and Western Digital had released the first 3 TB HDDs, with those companies and Toshiba producing the first 4 TB drives early the next decade. In 2013, Seagate had a 5 TB HDD, while HGST (a Western Digital subsidiary) shipped a 6 TB helium-filled hard disk.

Helium offers less drag and turbulence than air because it is less dense and lighter than air. That means drives filled with helium run cooler and faster and can have higher storage densities. In addition, a helium-filled hard disk enables manufacturers to put seven platters in the same space required for five platters in conventional hard drives.

Also in 2013, Seagate introduced hard disks using shingled magnetic recording (SMR) technology to further overcome the physical limitations of conventional drives. SMR layers magnetic tracks on each disk instead of placing them parallel to each other as in conventional hard disks, thereby increasing storage density. The tracks overlap like shingles on a roof, hence the name of the technology.

Today, thanks in part to the development of helium-based HDDs and SMR technology, hard disk capacities have grown to 10 TB, 12 TB, 14 TB and 16 TB.

Storage capacity chart

Parts of a hard drive

A hard drive consists of several major components inside its casing. These include the platter for storing data, a spindle for spinning platters, a read/write arm for reading and writing data, an actuator to control the action and movement of the read/write arm and a logic board.

Hard disks include one or more aluminum, glass or ceramic platters made of substrate material with a thin magnetic surface, or media layer, to store data. Platters store and organize data in specific structures -- tracks, sectors and clusters -- on this media layer, which is only a few millionths of an inch thick. A superthin protective and lubricating protective layer above the magnetic media guards against accidental damage and contamination by foreign material, like dust.

Components in a hard disk
Hard disk components

The spindle rotates the platters as needed and holds them in position. The rpm of the spindle determine how fast data is written and read. For multiple-platter HDDs, the spindle maintains the platters at a fixed, separated distance to give the read/write arms room to operate.

The read/write arm positions the read/write head over the correct places on the disk platter to access or write data. It is the read/write heads that read and write data to and from the platters by transforming their magnetic surface with electric currents. Typically, there is a read/write head floating 3 to 20 millionths of an inch above the top and bottom half of the surface of every platter side. All the read/write arms of a hard disk are fused together in the actuator motor.

The actuator motor receives instructions from the HDD circuit board and controls the movement of the read/write arm. It oversees platter data transfers and ensures read/write heads are always in the right place.

An intelligent circuit, or logic board, tells the actuator motor what to do. It is located at the bottom base of the unit, called the casing, and a flexible ribbon cable connects the circuit board to the actuator motor that controls the read/write arms.

Form factor

The entire disk must be mounted in an enclosure to protect the internal environment of the hard disk from outside contaminants and air. The drive's internals, also known as the head assembly, are mounted securely to the casing and then usually covered with aluminum. The form factor of an HDD is the size and shape of this enclosure.

The HDD form factor governs its compatibility with the drive bays of desktop and portable computers, servers, storage enclosures, storage arrays or any consumer product that uses a hard disk, such as a digital video recorder (DVR). Industry standards dictate the geometry of HDD form factors, which includes the length, width and height of the HDD, in addition to the orientation and position of the host interface connector.

Common enterprise-class HDD form factors are small form factor (2.5-inch) and large form factor (3.5-inch), with the measurements representing approximate diameters of the platter(s) inside the drive enclosures. While enterprise-class HDD enclosures typically have standard lengths and widths, height can vary -- up to 15 mm and 26.1 mm for small and large form factor enclosures, respectively. The 3.5-inch desktop form factor height ranges from 19.9 mm to 26.1 mm, while the mobile 2.5-inch hard disk's height ranges from 5 mm to 15 mm.

Hard disk destruction services

Just because data is deleted and is no longer accessible to the application or operating system (OS) that created it, that doesn't mean the information isn't available on a hard disk. Formatting a drive doesn't always destroy data bits, neither does overwriting data repeatedly with other data.

Specialized programs called hard drive shredders overwrite data and are intended to make that data irretrievable. There are experts who say original data may still be recoverable after using a hard drive shredder, even if the overwriting process used by these programs occurs hundreds of times over. Drilling holes through the hard disk won't necessarily do the trick either, as some tracks will remain unaffected.

The only way to ensure that all the data on a hard disk is destroyed is to pulverize the whole assembly. For a fee, companies such as ProShred and Securis will securely pick up and transport a hard disk and shred it much like a wood chipper disposes of brush and tree limbs. They will also certify the data has been destroyed in a manner that meets proper compliance and environmental regulations.

Common hard disk errors

Hard disks can fail for all sorts of reasons. However, failures generally fall into six broad categories.

Electrical failure occurs when, for example, a power voltage surge damages a hard disk's electronic circuitry, causing the read/write head or circuit board to fail. If a hard disk powers on but can't read and write data or boot, it is likely that one or more of its components has suffered an electrical failure.

Mechanical failure can be caused by wear and tear, as well as by a hard impact, such as dropping an external hard disk or the computer that houses an internal hard disk. This may cause, among other things, the read/write drive head to hit a rotating platter causing irreversible physical damage.

Logical failure results when the hard disk's software is compromised or ceases to run properly. All sorts of data corruption -- such as corrupted files, malware and viruses, the improper closing of an application or shutting down of a computer, human error or accidental deletion of files critical to hard disk functionality -- can lead to a logical failure. The effects of a logical failure vary from recurrent crashes to constant freezing and disk errors, the disappearance of data, inaccessible files and more.

Bad sector failure can occur when there is a misalignment of the magnetic media on a hard disk's rotating platter, resulting in a specific area(s) on the platter becoming inaccessible. Bad sectors are commonplace and often limited when they occur. Over time, however, the number of bad sectors can increase, eventually leading to a crash, inaccessible files or the hanging or lagging of the operation of a hard disk.

Firmware failure happens when the software that performs the maintenance tasks on a drive and enables the hard disk to communicate with a computer becomes corrupted or stops working properly. This type of failure can lead to the disk freezing during bootup or the computer a hard disk is connected to not recognizing or misidentifying it.

Multiple unknown failures that accumulate over time can also occur. For example, an electrical problem could lead to a mechanical failure, such as a read/write head crash. It might also lead to a logical failure, resulting in several bad sectors developing on the hard disk platters.

This was last updated in May 2018

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