Most non techy users concider the CPU, to be the PC's case. I have worked in IT for 10 years now, and ripped open lots of hard drives, and never found those magnets you speak of.. I even opened one up, poured Pepsi in it, and put it back together.. i reinstalled the drive in my friends PC.. needless to say it did not last too long..
Looks like there is more than one design to control and or move the arm in a hard drive.
The stepper motor design is actually an electric motor that moves from one stop position to another, governed by click stop positions. They cannot stop between stop positions. The motor is small and is located outside the HDA, so it is visible from the outside. The stepper motor design is inferior. It suffers from slow access rate and is very sensitive to temperature. It is also sensitive to physical orientation and can't automatically park the heads in a safe zone. Besides, the actuator operates blindly from the track positions, governed only by the stop positions. Over time, the drive becomes misaligned, requiring occasional re-formats to realign the sector data with the heads.
The servo motor actuator is another type of head actuator motor. Unlike the stepper design, the heads get feedback as to position, assuring proper tracks are read. The guidance system used by the heads is called a servo. Its job is to position the head over the correct cylinder. It does this through the use of grey code. Grey code is a special binary number system in which any two adjacent numbers provide info to the servo as to their position on the drive. Also, the heads are free to move wherever needed...no steps. Basically, when the drive needs to retrieve certain data, the servo motor moves the heads out to the appropriate position on the disk and then waits for the corrects bit of data to spin over to it. The time it takes for all this to happen is called latency, and is a key measure of the speed of the drive.
The voice coil is the latest type of device for controlling the heads and is used on all modern hard disks. The voice coil operates similarly to a speaker. There is a magnet (or magnets) that are surrounded by a spring-loaded coil of wire which is connected to the HGA. As a current is applied to the coil, it interacts with the magnet and swings the assembly. the resulting movement of the heads is from the center of the platters to the outer edges.
When the hard drive is powered down, the springs or actuator coil (depending on the type of actuator) attached to the heads pull the head into the platter. This is called a landing. Every drive is designed to handle thousands of takeoffs and landings, but since the head actually hits the platter, its best to have this happen on a section of platter where there is no data. In a voice coil design, the actuator coil springs the heads into a landing zone and lock position before the drive even stops spinning. The landing zone typically lies on the innermost cylinder or the outermost cylinder. This assures that the heads are not just let go of and left to drag along the platter until the platter stops, a problem common to the stepper motor design. When powered on, the drive automatically unparks itself and the coil is overcome by the magnetic force.
The spindle motor is responsible for spinning the platters. These devices must be precisely controlled and quiet. They are set to spin the platters at a set rate, ranging from 5400 RPM to 10000 RPM. The motor is attached to a feedback loop to make sure it spins at exactly the speed it is supposed to. The speed is not adjustable during operation. Some spindle motors are on the bottom of the drive, below the HDA, while the more modern ones are built into the hub of rotation of the platters, thereby taking up no vertical space and allowing more platters. Attached to the spindle motor is a ground strap which helps rid the drive of the static charges created by the rotating the platters through the air in the drive's interior. In many drives, this can be accessed by removing the logic board. After a while, this strap can become worn and produce noise, like a high-pitched squeal. One can usually lubricate the strap and stop the noise, but this entails some minor disassembling of the drive.