head crash - technical definition

The physical destruction of a hard disk. Misalignment or contamination with dust can cause the read/write head to collide with the disk's recording surface. The data are destroyed, and both the disk platter and head have to be replaced.

The read/write head touches the surface of a floppy disk, but on a hard disk, it hovers above its surface at a distance that is 5,000 times smaller than the diameter of a human hair. It has been said that the read/write head flying over the disk surface is like trying to fly a jet plane six inches above the earth's surface.