The accidental and disastrous contact of a read/write head with the surface of a hard disk as it rotates in a disk drive. Normally the head flies just above the surface. The disk has to be thrown away after a head crash as the head is much larger than the track spacing and the contact destroys the track so affected – and any data stored in that track and adjacent tracks. A head crash is often caused by the head passing over a dust grain on the surface. Particles of surface material produced by the contact cause other tracks to be destroyed.
The possibility of a head crash is reduced by keeping the disk clean and at a constant temperature and humidity. Disks are copied at regular intervals so that in the event of a crash a duplicate is available. Fixed disk drives have greatly reduced the probability of a head crash because they are sealed (except for a “breather” filter) and have an internal filter through which the air inside the drive circulates.