Re: Nice story, bro.
Really, because from the pictures it looks like it's made of cast iron. What did the disc packs weigh in that case?
That's the entire head positioner assembly. It's a LARGE (stationary) permanent magnet with the read/write heads moving on a linear guide, driven by a voice coil. It's quite similar to a loudspeaker.
The moving part of the positioner, as with the speaker, should be as low as possible so that the heads can move at the highest possible speed (so, shortest possible seek times), without requiring kilowatts of energy.