In Mac OS 10.4 Tiger a new feature was added to Disk Utility that writes random data to the free space on your hard drive. “Who cares,” you ask. Well, if you work with sensitive information of any kind, and delete it at any point, that data is still on your drive even though it does not appear to be available. Any technically inclined miscreant with physical access to your machine can recover that data if they had to…they’d just need a good bit of time, patience, and the right software (http://subrosasoft.com/OSXSoftware/index.php?main_page=product_info&products_id=1).)
By writing random data over the free space 35 times, Disk Utility’s “Erase Free Space” function effectively makes it impossible to recover deleted files. However, if you’re curious and want to try it out, it’s going to take quite some time—start it before bed and wake up the next morning to a finished process. If you cancel it, or force quit, you’ll be left with a drive with almost no free space. A frantic search of your hard drive for the gigantic file will turn up nothing, and you’ll be mad. Don’t fret—just take one file, any file, put it in the trash and empty it. Your drive will then show the correct amount of free space.
You can also empty the trash securely to begin with. From the Finder menu, select Secure Empty Trash. It takes forever, but the files will be completely destroyed.