Stepper motor
The open-loop mechanism that moves the head a fixed number of steps per track — simple, cheap and sensitive to heat.
A stepper motor turns a fixed angle per pulse, so the head can be counted out to the right track without any position sensor. That made it cheap, and it sat in practically every floppy drive and in early hard drives such as Seagate's ST-506 (1980). The drawback is that the control is open-loop: no feedback says where the head actually ended up, and when the metal warms and expands the tracks wander until the motor misses. That ceiling on track density was first broken by the voice coil motor with servo tracks, while floppy drives kept the stepper for life.