A head crash is a specific type of hard disk failure, and occurs when the read-write head of a hard disk drive touches its rotating platter resulting in catastrophic damage to the magnetic media on the platter surface (see Hard disk platter).
The head normally rides on a thin film of moving air entrapped at the surface of the platter (some drives of the mid-1990s used a thin liquid layer instead). The topmost layer of the platter is made of a Teflon-like material that acts like a lubricant. Underneath is a layer of sputtered carbon. These two layers protect the magnetic layer (data storage area) from most accidental touches of the read-write head.
A head crash can be initiated by a tiny particle of dirt or other detritus which can cause the head to bounce against the disk, destroying the thin magnetic coating on the disk. The disk read-and-write head is made using thin film techniques that include materials hard enough to scratch through the protective layers.
Since most modern drives spin at rates between 5,400 and 15,000 RPM, the damage caused to the magnetic coating can be extensive. At 7,200 RPM the edge of the platter is traveling at over 74 miles per hour (120 km/h), as the crashed head drags over the platter surface the read-write head generally overheats, making the drive or at least parts of it unusable until the heads cool. Following a head crash, particles of material scraped free of the drive surface greatly increase the chances of further head crashes or damage to the platters.
Older drives typically rotated far more slowly and had larger heads flying higher above the surface of the medium. However since in many cases the medium was housed in a removable cartridge or pack and since air filtration was comparatively crude crashes were fairly frequent and invariably expensive.
In large format printing the term head crash refers to the sudden jamming of the print heads on the scan axis. Typically the result of print media bunching up, a head crash can result in damage to the print head, requiring replacement.