HOWTO securely Erase whole harddrives
If you need to replace a computer then you probably want to make sure the information on the hard-drive is securely erased before you deliver it to your local recycling facility.
 Using dd
dd (manual) is a handy tool used for making copies. It can also be used to quite securely wipe harddrives:
This will overwrite your entire hard drive (in this case seen as hda by Linux) with random information ten times.
Thanks to Ritter and EnIgMa on linux-noob EFNet
 Using shred
shred (manual) is a tool designed to "overwrite a file to hide its contents, and optionally delete it". GNU/Linux considers block devices to be files, so you can also use it to wipe entire harddrives with a command like this:
The -n option tells shred how many times it should overwrite the harddrive. You may increase this number depending on time and paranoia. The -z option tells shred to overwrite with zeros to hide the shredding after it is done.
Thanks to Walt R.