Voice coil actuator
The moving-coil motor that swings the head-arm across the disk, named for the identical mechanism in a loudspeaker.
Early hard disks positioned heads with stepper motors that stepped to fixed track positions — fine until tracks grew too close and thermal drift made open-loop stepping unreliable. The voice coil actuator replaced them: a coil sitting in a strong permanent-magnet field pivots the arm when current flows, while a closed servo loop reads position information off the platter and corrects continuously. It is the same principle as the coil behind a loudspeaker cone, hence the name — fast, precise, and the source of the seek noise you hear. When power is cut, a spring or the coil's own back-EMF retracts the heads to the landing zone or onto the ramp.