the old "System Mechanic" software by iolo, inc used to write and overwrite data 7-9 times over with encrypted data...supposedly 7x overwrited was the government preference at the time (2000 I believe?)
they have a program called a "secure shredder" too which also destroyed data. don't forget though, the nature of magnetic discs...there will be traces of your data even if you smash it to pieces, or soak it in acid.
do what kevin mitnick did: throw your drives into a lake, never to be found.
~Branson