Strongeagle:
To be clear, the reason why it is possible to recover data from a hard disk is because the heads do not precisely track the same position each time that they pass over the disk. This leaves tiny "edges" where the heads wrote data last time but missed this time. This is what the forensics specialists recover.
In the early days, hard drives used stepper motors to position the heads and this was followed by voice coil actuators as density increased. In both cases, one could expect that data might be recovered, but modern high density drives have much narrower track spacing. They use very sophisticated techniques like piezo micropositioners on the heads to improve accuracy of the voice coil to lock on to the servo pattern. I would think it would be very dificult to recover prior data on such drives, even using custom test rigs. The latest enterprise drives have robust onboard encryption as well, which makes the problem even more difficult.
Take the lid off one of these drives and it's amazing how much tecnology goes into your $100 2Tb drive :-)...