A hard disk drive (HDD) is a data storage device used for storing and retrieving digital information using rapidly rotating discs (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 just sequentially. An HDD consists of one or more rigid ("hard") rapidly rotating discs (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 PCs. More than 200 companies have produced HDD units, though most current units are manufactured by Western Digital, Seagate, and Toshiba. Worldwide revenues for HDDs shipments are expected to reach $38 billion in 2012, up about 19% from $32 billion in 2011.
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 due to use by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. Performance is specified by the time to move the heads to a file (average access time) plus the time it takes for the file to move under its head (average latency, a function of the physical rotational speed in revolutions per minute) and the speed at which the file 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. Different sizes are used in specialty devices such as portable media players or in some server hardware. HDDs are connected to systems by standard interface cables such as SATA (Serial ATA), USB or SAS (Serial attached SCSI) cables.
As of 2012, 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 for the forseeable future due to advantages in recording capacity and price per unit of storage, but solid state drives are replacing rotating hard drives in portable media players, ultrabooks, and other portable electronics where speed, physical size, and durability are more important considerations than price and capacity.
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Famous quotes containing the words drive, hard and/or disk:
“Did I make you go insane?
Did I turn up your earphone and let a siren drive through?
Did I open the door for the mustached psychiatrist
who dragged you out like a golf cart?
Did I make you go insane?”
—Anne Sexton (19281974)
“Have you never been moved by poor mens fidelity, the image of you they form in their simple minds? Why should you always talk of their envy, without understanding that what they ask of you is not so much your worldly goods, as something very hard to define, which they themselves can put no name to; yet at times it consoles their loneliness; a dream of splendor, of magnificence, a tawdry dream, a poor mans dreamand yet God blesses it!”
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This maple burn itself away;
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Ray round with flames her disk of seed,
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With summer spice the humming air;”
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