Harddisks are very complex, high precision, electro-mechanical devices, containing high precision mechanical parts. Due to their electro-mechanical nature they're very sensitive to external shock and vibration. Indeed, dropping the harddisk onto a bench can damage the disk internally with no exterior evidence of damage. The first indication may be that it doesn't work when switched on. Even worse, the disk can suffer internal damage which does not lead to immediate failure but causes deterioration over time so that data integrity is threatened.

The harddisk is actually the most vulnerable when it is removed from the original packaging, which is carefully designed to protect it after leaving the closely controlled manufacturing environment. Once installed, it is relatively safe from any damage. The computer's casing typically absorbs any shock energy so that level experienced by the disk is very low. It is not surprising that the drive can be damaged when dropped onto a hard surface. But a comparably high level of shock can be produced just by tapping with a hand tool such as a screwdriver, by knocking two drives together, or forcing a drive into its slot.

The most common type of damage occurs when a harddisk is dropped or shocked and is called a 'head slap.' A head slap basically means that when sufficient shock is applied to a drive, the head lifts up off the disk, is whipped back down and bounces around on the surface of the disk. The result is that the head digs into the surface of the disk creating surface damage, and tiny particles or debris can be scattered. This all happens on a microscopic level, but the high precision nature of a harddisk means that the disk could fail at a later stage if some of the debris lands on a data zone or if the debris becomes wedged between the head and the disk.