Just a point of interest, from experience data can be recovered from a disk that has been formatted up to 6 times, I accept that this is a very specialised recovery process and beyond mr average, but it can be done. So format the drive a min of 7 times !!.
Alternatively open up the drive, have a look how it works then just doodle on the platters with a nail or screwdriver or similar. Only real way to destroy the data is to destroy the drive.