A hard disk is the main form of secondary storage, as opposed to RAM as primary storage. The hard disk stores all of the programs and data required by the computer and its user, e.g. the operating system. In comparison to RAM, the hard disk retains its data when power is switched off and its storage capacity is many times that of RAM, i.e. modern hard disks can store many gigabytes of data.
A modern hard disk is a complex, high precision device that contains a mixture of electrical and mechanical parts. The data is stored on both sides of rigid circular disks, i.e. platters. These platters are covered with a special magnetic coating. A typical hard disk has several platters mounted one above the other on a spindle which is used to rotate the platters at a high speed.
The platters of a hard disk have to be made to extremely precise specifications and are normally manufactured from an aluminium alloy.
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