The magnetic heads are combined into an assembly, and the assembly is attached to an actuator arm. The actuator arm holds all the magnetic heads and is moved across the platter(s), as a synchronous assembly. Originally, hard drives moved this assembly by borrowing concepts from audio speakers, called a voice coil. The voice coil method has the ability to use feedback for accurate placement of the heads. The definition for this feedback system is known as a servomotor. This was an expensive process.
Alan Shugart came up with the idea of a lower cost method, using a stepper motor. This is a motor with predefined steps. This caused issues with expansion and contraction of the materials used when temperatures changed! (Suddenly the data wasn't where is was.)
Advances led to the demise of the stepper motor, and back to the voice coil. As drive density increased, even servo based hard drives had challenges. The drive makers resorted to a process known as thermal re-calibration. This did not fare well for the new multimedia, Audio-Visual computing environment. Re-calibrating in the middle of a CD burn that took about an hour on blank media that was $20 a shot made for upset users. Some drive makers released AV rated drives. The issue of thermal re-calibration has largely been eliminated as an issue.
Now that the physical attributes of a hard drive have been revealed, its time to de-mystify how the drive operates, and with that, some new terms you may not have seen before.
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