The function of the motor is to spin the platters to the speed according to the drive's specification. Typical speeds on modern drives are 5400, 7200, 10,000 and 15,000 RPM. Depending on the type of drive and the application it is being used for. Servers would use fast drives such as SAS running at 15,000 where an entry- level laptop may be ok running at 5,400.
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.
Data recovery from drive with motor issues is possible but is the most specialised job within a data recovery business. Usually, motors can not be replaced without removing the platters. In some situations we remove platters and implant them into another drive shell with a functional motor. In other cases where platter alignment is so critical that they can not be removed, we have specialist equipment that can extract the bearing from the motor and replace it with a new one.
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.