Anyone reading this forum realizes that when you empty your trash that the files could still be recovered. If you really want to secure your Mac and clean your free space of any bits of the previous files that were there, you can with diskutil.
Disk Utility offers a one-line Terminal command to scrub free space of existing data. Execute diskutil secureErase freespace 3 /Volumes/name-of-drive. The utility will write to each drive sector 35 times, using a special algorithm. Thirty-five passes is well above the U.S. Department of Defense’s own standard for erasing data, which requires only seven passes.
