You actually have to overwrite something at least 7 times before it is totally gone. Any fewer than that, and high end equipment can pick up traces of data that has been overwritten. Think of it in terms of a chalk board. If you have a totally clean chalk board, then write a zero down, wipe out the zero with your hand, then write a one down in the same spot, you should still be able to see traces of the zero behind the one. A HD is the same. Even if you zero the entire drive once, high end equipment will still be able to get that data. You should overwrite it 7 times at least. More times if you really want to make sure it's gone.