To safely and securely erase your hard drive contents, you can use the shred command.
Boot up your system using a Knoppix Live CD and use the shred command as shown in the example below:
shred -vfz -n 100 /dev/hda
/dev/hda is the disk drive you wish to wipe and shred is making (-n) 100 passes by overwriting the entire hard disk with (-z) zeros. The shred program (-f) forces the write by changing the permissions wherever necessary.
