In case you want to auction or sell your Laptop or an old hard disk, make sure people have a hard time retrieving your data.
- Download SystemRescueCD, for example.
- Boot into Linux (could be you have to change the boot order in BIOS)
- Use # fdisk -l to list installed drives on the machine.
# shred -vfz -n 100 /dev/sda
Here /dev/sda (sda1, sda2 might indicate partitions only) /sda is the whole hard disk. Make (-n) 100 passes by overwriting the entire hard disk with (-z) zeros. And (-f) forces the write by changing the permissions wherever necessary. (via)