Meh I'm not that concerned with it.
People will tell you that you can repeatedly overwrite your drives (and I mean not just delete the files and write to that area of the drive, but the whole thing) and this will tend to cause the tiny magnetic crests on your drive (which represent bits) to gradually get "washed out", because each write is slightly imprecise, so it will sort of "smooth out" the area.
But this is still somewhat theoretical and there's no way for you to check that it's unreadable without industrial strength equipment.
A strong magnetic field might be useful. Any thoughts on that approach, Jack?