The controller can only handle data from one head at a time, which does not result in any loss of speed as the computer can only take in so much data at one time. There are three types of heads commonly used, composition, monolithic, and thin film. As a normal operation, a head landing occurs when the disk drive is turned on or off. When the heads land, a thin film of lubricant on the disk surface protects the disk from damage. A head crash occurs when the head and disk damage each other during landing because of rough handling or because a small particle gets between them.
Head crash is a catastrophic failure condition and causes permanent damage and loss of data.
In many Winchester drives, the head is in contact with the platter when the drive is not powered. This air bearing prevents any mechanical contact between head and disk. This is analogous to a plane taking off and landing. If the drive is jolted, data under the heads can be lost. The stepper motor drive must be parked through software which places the head at a safe zone.