sudo shred -v -z -n 1 /dev/sda
(where 'sda' is whatever your drive is - sda, sdb, sdc etc)
The parameters used in the above example are: -
- -v = verbose (show progress)
- -z = add a final overwrite with zeros
- -n 1 = Overwrite 1 time (instead of the default of 25)
This should overwrite any sensitive data on the disk once followed by zeros. Modern hard drives shouldn't need more than one pass overwrite anyway according to this article.
This method is by far the fastest that I have found - some of the other methods were estimated to take 3 days to complete on a 250GB hard drive, this way only took an hour or two.