more to add.
^^^ this, this and this again.. Hits the nail on the head. multiple Large, circular (or ark for doorways ;D ), alternating direction, HighFrequency&Amperage/Low Voltage AC , in bursts, alternating poles, through a silver or copper monofilament wire, degaussing stations are the only true way to fully corrupt the data through all levels without taking it apart and heating the disk to red hot. (just getting the disk red hot is more than ehough to make it forensically impossible to retrieve data)..
if one is truely paranoid, Cut the disks into 1/4s using a sawzall (and a blade for structural steel , you'll need it), Then, drill a hole in the center of all 4, and mig weld them together on the edges and through the holes... God couldn't retrieve the data off that