It might be difficult to control the heads if each one has other heads moving right next to it. Magnetic effects of the voice coils, mechanical vibration, maybe electrical even. It seems a much harder control problem given that the requirements are so stringent.
Rejection of neighboring HDD vibe disturbances is already an issue, so fascinating to see what their control approach is. In my limited experience, mechanical HDD control is near top of the game for electromechanical positioning.
At least they know what all the heads are doing so could design them to cooperate or model/predict the disturbances pretty well, unlike for external vibrations from neighbouring disks.
