Programatically Secure Erasing Free Space

One of those security things that pops up every now and then is to use the secure erase feature of Mac OS X, located in Disk Utility. But you can access this same feature from the command line using the secureErase option in diskutil followed by the freespace option. The format of the command is:

diskutil secureErase freespace [level] [device]

The levels are as follows (per the man page as not all of these are specified in Disk Utility):

1. Single-pass zero-fill erase
2. Single-pass random-fill erase
3. US DoD 7-pass secure erase
4. Gutmann algorithm 35-pass secure erase
5. US DoE algorithm 3-pass secure erase

So for example, let’s say you had a volume called Seldon and you wanted to do a standard Single-pass zero-fill erase. In this example you would use the following:

diskutil secureErase freespace 0 /Volumes/Seldon

If you were to automate the command then you would want to dump the output into a log file. For example:

diskutil secureErase freespace 0 /Volumes/Seldon > /var/log/secureeraselog.tmp

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