I've been wondering about this for a while, why do you have to overwrite data so many times to be completely sure a malicious person cannot restore your data? I would have thought overwriting the data once would be enough, but I guess the harddrive and disk is constructed so recovering of overwritten data is possible.

Earlier I heard overwriting data 7 times would be enough, then you would be really, really sure noone would be able to restore. Today I read that according to new standards for top secret data, it should be overwritten 36(!) times. Now that's a lot writing just to delete a few bytes!

Well what I don't understand you might have guessed, how is it possible to recover overwritten data? And why is it with the construction of media (harddisk, floppy, other removable media) that makes it nessecary to overwrite data so many times?

Information would be greatly appreciated!