How does oft-recommended hard drive scrubber Darik's Boot and Nuke actually work?
DBAN is simply a Linux-based bootable image that can be run from floppy, CD/DVD, or USB drive (and probably more) which runs in system memory from the media. Once started, it gives you the option of overwriting disk content with multiple different methods, including pseudo-randomly generated numbers, the Gutmann method, and the DoD methods. For the most thorough wipe the Gutmann method is preferred, but has the longest runtime (35 passes).
As opposed to a simple format which is recoverable, DBAN will actually overwrite each sector on the disk with gibberish. The amount of times and type of data depends on which method is selected as described above.
I use it at work and in a nutshell it writes the the hardisk with all 1's and then all zero's (It is the standard for the DoD???) and depending the option you choose it can do this a few times or several time rendering any info on the disk irretrievable. I work in government and it ensures that any PC that we have replace can be resold etc... with out the fear of any info being left on it.