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 Disc drive are  classified  nonvolatile, random  access,  bottom , magnetic, data storage device  and [http://harddriveenclosure.org/ Importance Of Using A Hard Drive Enclosure]

Introduced  by IBM in 1956 hard disk  drives have decreased  in cost and physical  size over the  yrs while dramatically increasing in capacity and speed. Hard disk drives have been the  prevalent  device for secondary storage  of data in general  purpose  computers since the early 1960s. They have maintained this  perspective because advances in their  transcription capacity, cost, reliability, and speed have kept pace with the  necessaries for secondary storage . A  hard disk  records data by magnetizing a thin film of ferromagnetic material  on a disk. User data is encoded into a run-length limitedcode and the encoded data written as a pattern of sequential magnetic transitions on the disk. The data is represented by the time between transitions. The self-clocking nature of the run-length limited codes used enables the clocking of the data during reads. The data is read from the disk by  discovering  the transitions  so  decoding the written run-length  restricted  data back to the user data.

The heads  are kept from  adjoining  the platter  area  by the air that is extremely  more or less  the platter; that air  goes at or near the  disk  speed. The record and playback head are mounted on a block called  a slider, and the surface next to the platter is shaped to keep it just barely out of contact. This forms a type of air bearing. Opposite the actuator at the end of the head support arm is the read-write head (near center in photo); thin printed-circuit cables connect the read-write heads to amplifier electronics mounted at the pivot of the actuator. A flexible,  more or less U shaped, ribbon cable, seen edge-on  sub contract  and to the left of the actuator arm  goes on  the  link with the  comptroller  display panel  on the opposite side of [http://harddriveenclosure.org/importance-of-using-a-hard-drive-enclosure/ How to assemble a Hard Drive Enclosure]