I just purchased a rather larger (1″ dia, 3/4″ thick) N52 super magnet to erase MiniDV video tapes. Wow! What an amazing thing. I can see why I had to agree to all the safety notices on the item. If you get your finger pinched by it, it would do some damage. Not finger severing, but it would certainly bring up a nice blood blister!
This thing is so strong you can hang a few pounds from it with only the contact with a wallboard screw! And yes, with a few swirls, a MiniDV tape is completely erased. For those interested, K&J magnetics is the place to go.