Well, using tools such as DBAN or the like is considered to be mostly pointless and also very time consuming.
Generally, you do not need to do anything, but fill the drive with 0x00 (zero bytes / NUL) only once these days, in order to securely prevent the recovery of former data.
Doing multiple passes is excessive and mostly useless, let alone filling the drive with random data. The only way you can try to restore anything, after such an operation took place, is with an atomic-force microscope - this is, obviously, an extreme procedure, that will take months for even the smallest JPG file and the error rate (false-positives) is going to be immense (in other words - you won't get anything meaningful out of it). This is even more true for higher capacity models (higher density platters).
However, one can only speculate what tech might be in the hands of, say, the NSA, so judge the provided information with that in mind.
So, one of the ultimate software ways (fast, reliable & secure), is a single run (zero-fill) of dd:
dd if=/dev/zero of=/dev/sdX bs=1M
or, if you want to measure progress:
pv < /dev/zero > /dev/sdX
However, there is something called secure erase. This is an established ATA standard. This functionality is integrated into the drive itself. Not only is it even faster than a run of dd (as it's already hardware based and hardware > software, speed wise), it is also more secure, due to the ability of purging original bad sectors that have been reallocated! There are 2 versions: the vanilla (2001 and onwards) and the enhanced one (post 2004). So, if your drive was manufactured roughly 10 years ago - it is already very likely to support this feature.
hdparm --security-set-pass NULL /dev/sdX
hdparm --security-erase NULL /dev/sdX
hdparm --security-erase-enhanced NULL /dev/sdX
Erase away!
@Chandrasekar: Simpler and ineffective. Please read any of the answers in the IT Security post that Shadok linked to understand why. – Ben Voigt – 2016-01-22T16:17:49.057
1Sorry, I thought your wording might have implied you weren't sure. I mostly wanted to make it nice and clear for anyone else reading :) – Blorgbeard is out – 2009-08-02T11:14:35.907
3Note that a reformat is indeed absolutely trivial to recover data from. You should definitely run some kind of wipe utility (that I'm sure other answerers will suggest :) – Blorgbeard is out – 2009-07-16T06:07:11.980
4Hence why I asked the question – Josh Hunt – 2009-07-16T06:08:09.513
Here is a recipe for the Macintosh that doesn't involve destroying, or even formatting your hard drive: http://macmad.org/blog/2010/10/clean-private-data-before-selling-a-mac/
– Jamie Cox – 2011-01-20T14:49:45.1371
Here is a post on IT Security talking about that: http://security.stackexchange.com/questions/5749/how-can-i-reliably-erase-all-information-on-a-hard-drive
– Shadok – 2012-03-21T09:42:22.443Another simpler way will be to format the drive and copy some junk files and fill the HD completely. And may be format again and fill it with some other junk files. So that even if someone use recovery software, they can only recover the junk data you filled recently. – Chandrasekar – 2014-04-01T02:12:41.320