If I were tasked with cleaning up after something that required disk destruction, I'd order a bunch of new preinstalled-windows disks first, swap them out and then disassemble and grind off the magnetic surfaces with a disk-grinder before discarding the old disks.
Years ago, when the SAGE air defense system used drum memory units manufactured by Ferranti, they would be resurfaced periodically by being sent to Toronto - handcuffed to a human guard. The drum would be turned down on a lathe and the shavings collected into a bag by the human guard, and the drum then returned back into service. I guess the bag thing was to ensure that no one could reconstruct the information from the shavings, which seems pretty improbable. Peter -- Peter Hiscocks Syscomp Electronic Design Limited, Toronto http://www.syscompdesign.com USB Oscilloscope and Waveform Generator 647-839-0325