Best way of making sure no data is recovered.
1. take hard drive out of computer
2. Lay it on a flat hard surface
3. Take hammer and smash it into dust.
4. then dump into salt water.
5. dump it's pieces in different places.
That and only that will make sure it is never found on your computer.
Data recovery is so far ahead of the game that no program will delete it. Don't think for a moment it is. I don't care what you want to delete old pictures or tax returns Never give a hard drive away with the computer, never think that hard drive is dead and put in the trash, make sure take apart a burn the disk in. Now for flash memory it is in old cell phone or camera same hold true. It can be recovered.