In order to wipe out data on a hard drive you can download free software which will overwrite every sector with zeroes. Western Digital's Data Lifeguard can be downloaded here. Choosing the option to 'WRITE ZEROS' will make it impossible to recover data with forensic tools.
I ran the Data Lifeguard utility on a flash drive, from which I had previously recovered deleted files with Test Disk. See the tip of the night for January 31, 2016. Afterwards JPEG files that were recovered by Test Desk could no longer be recovered with it.