Recently I found out about a nice feature of Mac OS X’s Disk Utility: The ability to securely delete unused space. Huh? – you might say – why do that? Well, one nice feature of the Finder is that you can securely empty the trash. This not only removed information about the file from the file system’s directory (e.g., file allocation table), but actually overwrites the data on the hard disk such that it cannot be recovered, or at least, to make recovery more difficult.
Sometimes I forget to choose secure empty trash, and once the file has been deleted its too late to securely delete it. That’s were “Erase Free Space” comes in. In disk utility, select the partition (not the drive), then the Erase tab, and then choose “Erase Free Space”. You get three options: single, 7-fold, and 35-fold overwriting. For a 250 GB HDD, single erase takes about 2-3 hours. 7-fold erase takes 7 times as long, i.e., 14-21 hours. 35-fold erase seems rather impractical. What if you have a power failure during those 3-4 days it takes?
It’s certainly easier to just use Secure Erase Trash, but now knowing about the Erase Free Space, the question is, how many times does dat get written over? Apple’s help page does not tell. This discussion on Mac OS X Hints sheds some light on the details. One post concludes that secure empty trash does a seven-fold overwrite. Also interesting is that secure empty trash makes use of an underlying terminal command – srm – which stands for secure rm (remove).