Hard Disk Drive Definition

A hard disk drive (HDD) is a type of electromagnetic data storage device that features an outstanding combination of high capacity, high speed, high reliability, small size and low cost. Thus HDDs serve as the main storage device on most computers as well as for a growing number of other products, including video surveillance equipment, scientific instrumentation, cameras, set top boxes for televisions, satellite TV receivers and portable music players (such as Apple's iPod).

Storage in a computer context refers to devices or media that can retain data for relatively long periods of time, for example, years or even decades. This contrasts with memory, whose contents can be accessed (i.e., read and written to) at extremely high speeds but which are retained only temporarily (i.e., while in use or only as long as the power supply remains on).

A HDD consists of a rigid metal case that contains one or more identical platters, at least two magnetic heads for each platter, a spindle motor for rotating the platters, an actuator mechanism for moving the heads, and control circuitry.

A platter is a thin, high-precision aluminum or glass disk that is coated on each side with a a very thin layer (typically only a few millionths of an inch thick) of a high-precision magnetic material in which the actual data is stored. Data is written to and read from this coating by magnetic heads, which are highly sensitive and high-precision electromagnets. There is usually one head for each side of each platter.

The magnetic coating on each side of each platter is divided into tracks. A track is any of the concentric circles on the magnetic media over which one head passes while the head is stationary but the disk is spinning. Each track on a modern HDD has a width of only a few microns (i.e., millionths of a meter), and there can be tens of thousands of tracks on each platter. The thinner the tracks, the greater the storage capacity of the disk. A single, imaginary, concentric circle that cuts through all of the platters and includes the same track on each side of each platter is called a cylinder.

Tracks are divided into a number of segments called sectors. Each sector generally contains 512 bytes and is the smallest unit of data that can be accessed by a disk drive (although software makes it possible to access individual bytes and even individual bits). The operating system keeps track of where data is stored by noting its track and sector numbers.

Most HDDs are designed to be used inside of computers or other products. They are mounted in a dedicated slot in a computer and connected to the power supply and data circuits via flexible cables. There are usually at least two such slots in modern personal computers so that computers can contain multiple HDDs if desired. External HDDs are generally more expensive than comparable internal HDDs but have the advantage of greater flexibility.

The first commercial HDD was launched by IBM in September 1956. Designated the IBM 350 RAMAC (random access method of accounting and control), it had a capacity of about five megabytes, which was attained by using 50 platters, each 24 inches in diameter. The average data access time was very slow by today's standards, largely because there was only a single head to access to all the platters. IBM's 3340 Winchester disk system, introduced in 1973, was the first HDD to use a sealed head/disk assembly, which substantially improved performance and is now standard.

The history of HDDs has been one of continuous rapid progress, particularly with regard to capacity, reliability, miniaturization and cost reduction. This has been the result of advances in a number of areas, including the development of improved magnetic media with greater areal density (i.e., increased data storage capacities per unit of area), increased precision of the heads, motors and other mechanical parts, and improved control circuitry.

For example, in 2004, Toshiba Corporation introduced the first 0.85 inch (two centimeters) HDD, and it began shipments of 2GB (gigabyte) and 4GB models in 2005. Until that time the smallest HDD was 1.8 inches.

In April 2006 Seagate Technology LLC introduced a 3.5 inch HDD for desktop computers with a capacity of 750GB, which is sufficient to hold roughly 75 hours of high-definition television programming or the amount of data contained in books produced from thousands of trees. This is an increase of 50 percent from the previous industry maximum and is said to be the biggest jump in HDD capacity in history. Moreover, the company also expects to offer terabyte (i.e., 1000 gigabyte) models in the future.

This progress is expected to continue for some time into the future. A particularly promising technology is perpendicular recording, which utilizes magnetic materials in which the individual bits are aligned perpendicular to the platter, rather than parallel to it as is the case today. Perpendicular recording is what made Seagate's new record possible, and it is expected to eventually result in areal densities possibly as much as ten times those in currently available conventional HDDs.

There has been much interest for a number of years in replacing HDDs with solid state storage, mainly flash memory, because solid state devices feature huge advantages with regard to weight, power consumption, shock resistance and longevity. In contrast of conventional memory, flash memory retains its contents even in the absence of a power supply.

However, because the cost on a per-bit basis is still far higher for flash memory than for HDDs and substantial improvements are continuing to be made in HDD capacity and performance, no large-scale replacement is likely for a number of years. Rather, replacement will continue to be mainly for applications for which miniaturization and durability are more important than price, such as ultra-portable computers, portable music players, scientific instrumentation and military equipment.

HDDs acquired the hard in their name from the fact that the magnetically coated platters inside of them are rigid, in contrast to the very flexible nature of the first floppy disks.

Created May 16, 2006.
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