I onced worked in an area where "Tempest" secure devices were dealt with. Hard drive failures required complete disassembly and then the disk (platen) was then held to a belt sander. Once the surface was obliterated, the disk was put in a vice and folded into quarters. A small vehicle with a flashing light would arrive every once in a while and collect the scruntched disks. The next step was out of the area but they were eventually smelted. Not being a security expert, I would say there would litlle or no chance of any data recovery.