BOAC, this is an excellent app to sterilise your HDD
Hard Drive Eraser . Meet US DOD specs with one setting and the creator has his own special setting the makes the DOD setting look weak. I have used this several times and I have never been able to recover old data using some of the better commercial data recover programs. As advised above, remove HDD from computer and run from a different machine. Make sure both devices have sufficient ventilation/airflow as the erasee drive does tend to get a tad warm due to the activity involved.