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A hard disk drive (HDD) is a data storage device used for storing and retrieving digital information using rapidly rotating disks (platters) coated with magnetic material.An HDD retains its data even when powered off. Data is read in a random-access manner, meaning individual blocks of data can be stored or retrieved in any order rather than sequentially. An HDD consists of one or more rigid ("hard") rapidly rotating disks (platters) with magnetic heads arranged on a moving actuator arm to read and write data to the surfaces.
Introduced by IBM in 1956, HDDs became the dominant secondary storage device for general-purpose computers by the early 1960s. Continuously improved, HDDs have maintained this position into the modern era of servers and personal computers. More than 200 companies have produced HDD units, though most current units are manufactured by Seagate, Toshiba and Western Digital. Worldwide disk storage revenues were US $32 billion in 2013, down 3% from 2012.
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 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. Performance is specified by the time required to move the heads to a track or cylinder (average access time) plus 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 in desktop computers and 2.5-inch in laptops. HDDs are connected to systems by standard interface cables such as SATA (Serial ATA), USB or SAS (Serial attached SCSI) cables.
As of 2014, the primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs). HDDs are expected to remain the dominant medium for secondary storage due to predicted continuing advantages in recording capacity, price per unit of storage, write latency and product lifetime. However, SSDs are replacing HDDs where speed, power consumption and durability are more important considerations.
Storage Capacity
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The capacity of a hard drive is measured in bytes. Modern drive capacities are in the gigabyte (billions of bytes) and terabyte (trillions of bytes) range and likely to go higher. The capacity is a factor of the number of platters, or disks, that are installed in the drive and the density of the magnetic storage capability of those platters.
Access Speed
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The hard drive is an electro-mechanical device. The data that is stored on the magnetic platters is read by a head that floats just above the surface as the disk rotates beneath it. The read-write head must move to different parts of the platter as it spins to read all of the parts of a file. The combination of the speed of the head movement and how quickly the platter can rotate under the head form the basis for the access speed.
Form Factor
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Early hard drives were huge, housed in separate machines and connected to the CPU via heavy cables. Modern hard drives are limited to three physical formats: 3.5-inch, 2.5-inch and 1.8-inch. The smaller physical size limits the number of platters and the diameter of those platters. A 1.8-inch drive, for example, has a maximum capacity of 320 gigabytes.
Interface
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The electronic connection between the hard drive and processor has undergone a number of changes over time. Each interface change has improved the data transfer speed and ease with which the hard drive is handled by the motherboard in the computer. The current standard interface is SATA, the Serial Advanced Technology Attachment.
