Hi there
You need some utility that does a PHYSICAL WRITE of x'00' (Binary zero's) on EVERY SECTOR.
Formatting or even wiping a partition won't do as this simply alters the directory contents but doesn't destroy the data. (If you don't like X'00' just write random Hex data but it must be on every sector).
Think of typical OS formatting and erase commands as simply tearing the Index and Table of Contents from a Book -- you can still READ the entire book as the actual DATA hasn't been destroyed.
This utility (has Military Strength erase) is quite good and FREE.
Disk Wipe - Free software
Cheers
jimbo
Try using something like this.