Ah yes, the old "bulk-eraser" trick? If you are fortunate enough, you can find a very powerful magnet inside a subwoofer! Other uses for such powerful magnets are to create magnetic field antennae for tv or radio. Oh, ah subwoofer should have at least 6 ohm written on it. Make sure the speaker can at least pick up a screwdriver, or if you stick it on your fridge you find it slightly difficult to take it off. But yes, no garauntee that will work to fully erase the data. I know commercial bulk-eraser does not even have a 100% success rate either, but if you go with a service like was mentioned before, they may have some sort of gaurantee.