I often get asked the best way to securely erase a disk or what to use to ensure that a disk has been sanitised to DoD 5220-22-M. A reasonable way to achieve this would be:
dd if=/dev/urandom of=/dev/rdsk/c#t#d#s2 bs=10k
or
dd if=/dev/arandom of=/dev/r#d#c bs=10k
This will ensure that random (not completely random, but good enough) data is written to the devices RAW geometry. This would remove all traces of previous data on the disk1
Oracle (aka Sun Microsystems) have provided a utility in the Solaris Operating System’s command format(1M) called analyse. Basically, you load the disks defect list, analyse and then purge.
Old documentation of the DoD 5220-22-M refer that the character, compliment, random and verify method can be used on data that is classified less than Top Secret. However, in the most recent versions of the [NISPOM] standard, this has been removed and only degaussing and/or physical destruction is valid for the sanitisation of hard disks – disks are cheap these days.
The Gutmann hypothesis has never been proven or subject to peer review and the triple write method of sanitisation has even been labelled by some as an urban legend. There have also been book/paper written discussing it [Overwriting Hard Drive Data: The Great Wiping Controversy].
So basically, overwriting the disk will be sufficient for most people and organisations, no matter what governance policy they come under2. For those that fall outside of this, find tin foil hats attractive or basically think we never actually walked on the moon, you are going to be better off breaking out the oxy acetylene kit and turn your disk into pulp.
1 As the drives controller maintains the bad sector tracking, any data written into bad sectors previously may still be there though the quality of this data would be questionable otherwise the sector would never have been marked bad in the first place.
2 If you are a classified government agency or your data is at a specific government data classification, a wipe will be out of the question. Remember, for you, disks are cheap.