Hard Disk Motors

http://www.pcguide.com/ref/hdd/op/spin.htm

All PC hard disks use servo-controlled DC spindle motors. A servo system is a closed-loop feedback system; this is the exact same technology as is used in modern voice coil actuators, and I discuss how servo systems work in detail in that section. In the case of the spindle motor, the feedback for the closed-loop system comes in the form of a speed sensor. This provides the feedback information to the motor that allows it to spin at exactly the right speed.

• Old Hard drives used a stepper Motor
• Stepper motors are also commonly used for both turning the spindle and positioning the head on floppy disk drives. If you have a floppy drive, find one of its motors and turn it slowly with your hand; you will feel the discrete step-wise nature of its motion
• The actuator in a modern hard disk uses a device called a voice coil