In audio recording which uses magnetic media similar to disks the only way to thoroughly erase a tape is with a so-called bulk eraser. This applies a very strong a.c. magnetic field and then is gradually withdrawn from the tape to allow the residual magnetism to "follow the hysteresis curve" down to zero. This would be difficult to apply to disks except floppies. Consider also the technique used in the Norton Utilities program "Diskwipe" which takes a /g switch which "Follows certain government rules for wiping". I can't find the manual but I think it specifies how many repetitions are used and different values to write in each.