Hard disk Drive

magnetic head

A hard disk drive (HDD; also hard drive, hard disk, or disk drive) is a device for storing and retrieving digital information, primarily computer data. It consists of one or more rigid (hence "hard") rapidly rotating discs (platters) coated with magnetic material, and with magnetic heads arranged to write data to the surfaces and read it from them.

Hard drives are classified as non-volatile, random access, digital, magnetic, data storage devices. Introduced by IBM in 1956, hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s. They have maintained this position because of advances which have resulted in increased recording capacity, reliability, and speed, as well as decreased cost, allowing them to keep pace with ever more demanding requirements for secondary storage.

History hard disk drives have changed in many ways.
>Capacity per HDD increasing from 3.75 megabytes to 4 terabytes or more, more than a million times larger.
>Physical volume of HDD decreasing from 68 ft3 or about 2,000 litre (comparable to a large side-by-side refrigerator), to less than 20 ml (1.2 in3), a 100,000-to-1 decrease.
>Weight decreasing from 2,000 lbs (~900 kg) to 48 grams (~0.1 lb), a 20,000-to-1 decrease.
>Price decreasing from about US$15,000 per megabyte to less than $0.0001 per megabyte ($100/1 terabyte), a greater than 150-million-to-1 decrease.
>Average access time decreasing from over 100 milliseconds to a few milliseconds, a greater than 40-to-1 improvement.
>Market application expanding from mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content.
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