Hard disk motor
Most motors use a feedback-circuit so the controller 'knows' when the drive is running at the correct speed before moving the heads over the data area on the platters. One-way of achieving this is for the drive to send a 'sync signal' or pulse each time the motor completes a single revolution. This pulse is received by the electronics on the controller card and the rotational speed is calculated according to the time between pulses. Once the speed is within the specified range, then continuous speed is upheld by precise adjustments sent to the motor.
Modern hard disk manufacturers no longer use standard ball-bearing assemblies in their motor bearings, and choose to use a magnetic lubricant instead. This type of bearing is known more commonly as a Fluid Dynamic Bearings (FDB). Because the lubricant is magnetic, they have much greater wear resistance as the lubricant is in contact with all moving parts and added advantage is that they run considerably quieter than older drives.
Symptoms of a seized motor can involve a 'creaking' type noise when powered up but no spinning. We see several drives with these symptoms every week and there is an excellent chance of recovery.