Dban is the best for what you want to do. It does take some time to get the job done, but if you Dban it, you don't have to worry about the average "Joe" taking a look at your information. If interested, then here is the link:
Dban Complete Disk Eraser
Please note: Be sure you have all disks unhooked that you do not want formatted and cleaned, because this program while great has the capability to erase multiple disks at one time if hooked to the computer.