I'm not sure you need to detect the head position, because in the end you need to deliver the appropriate signal at the same location, so it'd be simpler to jus

I'm not sure you need to detect the head position, because in the end you need to deliver the appropriate signal at the same location, so it'd be simpler to just have a linear array of 80 "heads" in the "disk" and let the drive head read from whichever one it likes. Then you would only need to track the spindle position. Head-to-head coupling works in the real world, as anyone who has used a cassette tape adapter in their car stereo knows. I don't know how much "horizontal" tolerance you would need in terms of emitting the signal from the "disk"; if different drives have their heads not quite on center it could present some difficulty (whereas with real disks it would not matter--they could position the head anywhere within the shutter aperture). Miniaturization would be another challenge--unless you're willing to have a ribbon cable coming out the front of the drive, you'll need to cram some batteries inside what is quite a thin space. Perhaps harvest some energy from the spindle (it would almost certainly be enough if you manage to find or create a thin enough generator). The last piece of the product puzzle might be to add Bluetooth...if for no other reason than being able to get data out as soon as it is written.