read or write

Disk read/write heads are the small parts of a disk drive which move above the disk platter and transform the platter's magnetic field into electrical current (read the disk) or, vice versa, transform electrical current into magnetic field (write the disk). The heads have gone through a number of changes over the years.
In a hard drive, the heads 'fly' above the disk surface with clearance of as little as 3 nanometres. The "flying height" is constantly decreasing to enable higher areal density. The flying height of the head is controlled by the design of an air-bearing etched onto the disk-facing surface of the slider. The role of the air bearing is to maintain the flying height constant as the head moves over the surface of the disk. If the head hits the disk's surface, a catastrophic head crash can result.

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