Zero-Out Free Space

When files are normally deleted they are just removed from the filesystem. The file itself is still on the hard drive though, which is how some programs are able to "recover" deleted files. For security reasons you may want to zero out your free space, which will get rid of all chances of recovering deleted files.

First open Disk Utility in /Applications->Utilities. Then select the hard drive partition on the left of the screen, select the erase tab, then click "Erase Free Space". Choose which option you want (the first option is usually enough) and click the "Erase Free Space" button. The process can take anywhere from an hour to quite a few hours depending on your hard drive size, and the amount of free space.

During the process a "You are running low on disk space" warning may pop up, just click OK on this and ignore it. That message is suppose to pop up. Disk Utility zeros out the free disk space by simply making a giant file that is nothing but zeros.

Warning: If you become impatient or worried and cancel the process when it is being done, you may have to restart and empty your trash in order to recover disk space. This will delete the giant file Disk Utility creates in order to zero the disk space. Despite what numerous forums claim, this process is very safe, and will not harm your data under normal circumstances.
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