SYSTEMS TODAY

Within a hard disk is one or several plates (usually between 2 and 4, although there are up to 6 or 7 dishes), which are disks (aluminum or glass) and concentric rotating all at once. The head (read / write device) is a set of vertically aligned arms that move in or out as appropriate, all at once. At the tip of these arms are the heads of read / write, thanks to the movement of the head can read both indoor and outdoor areas of the disc.
Each plate has two sides, and we need a head read / write to each side (not a head plate, but one per side). If you look at the schema-Cylinder Head Sector (below), at first glance are 4 arms, one for each dish. In fact, each arm is double, and contains 2 heads: one to read the face of the plate, and another to read the lower side. Therefore, there are 8 heads to read 4 plates. Heads reading / writing never touch the disk, but that pass very close (up to 3 nanometers), or 3 millionths of a millimeter. If one comes to him, causing extensive damage to the disc, rayándolo seriously because of how fast you spin the plates (one of 7,500 revolutions per minute moving at 120 km / h on the edge).