even format won't do much about it

If you want to totally wipe out your data, just format won't help you even low level format.

You have to create a big junk partition that is close to the size of our hard drive, and then format it. In this way, your free space will be real junk to new suspect users. Truecrypt help you to create that big junk.

I like sweet too, so you can create a sweet junk.