first – determine the dev of the hard drive – as in /dev/sda /dev/sdb etc

Then excute

shred -vz -n 3 /dev/XXX

where XXX is the identifier of the hard drive
Increase 3 to a larger value for higher security – but will take longer
could take up to an hour when using 3 – longer w/ a higher number

WARNING – putting the wrong value in for XXX will result in the wrong hard drive being wiped!!!!