Reader Steve Carter would like his deleted files to be really, really gone. He writes:

How do you delete a file completely and forever? I thought the original still remains on the hard drive until it is overwritten. And is there a way to go back and permanently erase old previously deleted files?

We’ll start with the basics. When you toss a file into the Trash and then empty it, the file isn’t really gone. It’s simply removed from your Mac’s file directory. With the proper recovery tools (Prosoft Engineering’s $99 Data Rescue 3, for example) you can recover it provided that you haven’t added other files to your Mac that have overwritten the portion of the drive that file once occupied.

To delete a file so that it’s unrecoverable by all but those who make their living recovering data, toss the file into the Trash and instead choose Finder -> Secure Empty Trash. When you do this not only is the file removed from the Mac’s directory, but the hard drive space it occupied is overwritten with junk.

via How to securely delete files | Macworld.