HDD Magnetic Disk
The magnetic disk is a round plate made of light alloys based on aluminum or, in rare cases, of ceramics or special glass with a cobalt layer put on their surface by a vacuum sputtering for giving magnetic properties to the disk. The technology of sputtering of a magnetic disk is similar to one used for producing integrated microcircuits. The magnetic covering of a disk is a set of the smallest areas named domains which change a magnetization vector under the influence of the external magnetic field created by magnetic heads at a data recording. After the termination of an external field influence on the disk, the zones of residual magnetization are being formed on the disk surface. That is the way the information recorded to the disk is being preserved. Areas of residual magnetization that appeared to be opposite to a magnetic head during the rotation of the disk direct electromotive force in the head, thus enabling reading the information back. Several plates of the magnetic disks planted on a spindle motor shaft can be set inside the HDD. The number of working surfaces, accordingly, is twice more (two per disk). Although contradicting computer science basics, manufacturers and sellers of hard drives have proclaimed that one gigabyte of information contains 1000000000 bytes of the information instead of 1073741824 that allowed them to "increase" the marketing capacity of disk drives by 7 percent.