Secure File Deletion

During the recent Windows unpleasantness I borrowed a couple of old disk drives to backup data on the “broken” disk — it was only broken under Windows, I could read it quite easily in Linux, but never mind. Now that everything's fixed, I want to give the drives back. The problem is that they have personal data on them, so I want to delete it all before I give it back.

This isn't as easy as it seems, since files leave traces all over. Fortunately, the folks at the Stanford Linear Accelerator Center (SLAC) have put together a guide to Secure Erase in UNIX, which also works in Linux.

I'm not going to repeat all the article. Just note that you need:

• shred, which does secure file erasure by repeated rewrites followed by deletion, and comes by default in Ubuntu
• scrub, which erases the free space in your file system by filling it up with a large, random file, which you then delete (with shred, if necessary). This isn't a standard file, you have to download and compile it.

Ideally, one would delete everything on the disk in question in the following way:

$ cd /media/disk # or wherever the thing is
$ sudo rm -fr * # delete all files and directories recursively
$ scrub -X junk # Fill up the whole disk with junk
$ shred --remove junk # rewrite several times, then delete

The difficulty is that the disk I used has a VFAT file system, so the biggest file that can be written is 4 GB. I just wrote

$ scrub -X junk
$ scrub -X junk1
$ scrub -X junk2

etc., until the disk was filled up, then ran

$ shred --remove junk*

If you are using a journaling file system other than ext3, there are some other issues involved, but for VFAT or ext3 this is a good way to go.

26 July 2009: Replaced the old link to scrub, which generates a 404 error, by the link https://computing.llnl.gov/linux/scrub.html, as noted by the first commenter. You can also download scrub directly from sourceforge at http://sourceforge.net/projects/diskscrub/.