dd is a great tool to overwrite disks quickly to erase all information on it (if you want to return them on warranty or sell them)1)
Overwrite entire disk, e.g. /dev/sdX with zeroes, for example to sell the disk or send it in for warranty. Be VERY CAREFUL as overwriting the disk will permamently destroy all data on it.
CAUTION: Only do this if you know what your are doing. Do this at your own risk!
0. DON'T do this in a hurry
1. Verify that you have a backup of all your data!
2. Locate the disk, make 100% sure by checking the size and the model!:
ls /dev/disk-by-id -l
3. Verify that you are on the right disk (check paritions):
4. Verify that the right hd light is lighting up if you read the disk
cat /dev/sdX > /dev/null
5. Overwrite the disk with zeros:
dd if=/dev/zero of=/dev/sdX bs=4096
6. Check the progress of your running dd, from another terminal window do 2)
watch killall -USR1 dd