You cannot destroy everything. I read an article in a magazine a while ago, I know you cant always believe them but the only way you can totally erase everything that was on there is by using a certain tool. Even then you have to run it 2 times to get everything.
The FBI has one and when they destroy a harddrive they send it through SEVEN times. Some may see this as overkill but when you are in their game you cant afford to take chances.