HARD DISK

A hard disk drive^[2] (often shortened as hard disk,^[3] hard drive,^[4] or HDD) is a non-volatile storage device that stores digitally encoded data on rapidly rotating platters withmagnetic surfaces. Strictly speaking, "drive" refers to the motorized mechanical aspect that is distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.^[5][6]
HDDs record data by magnetizing ferromagneticmaterial directionally, to represent either a 0 or a 1 binary digit. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle that holds one or more flat circular disks calledplatters, onto which the data is recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic material, typically 10–20 nm in thickness--for reference, standard copy paper may be between 0.07 millimetres (70,000 nm) and 0.18 millimetres (180,000 nm) thick.^[8]--with an outer layer of carbon for protection. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.^{[citation needed]}