ok, here goes, if you type wrong drive number you will wipe wrong drive beyond easy repair, so take care.
first you become "superuser" or more correctly root.
everything you do from now is dangerous.
first you find name of drive by typing on a commandline.
the outout wil be something like
(yeah I am using a pen-drive just now)
Disk /dev/sdb: 7948 MB, 7948206080 byte
Enhet start end Block Id System
/dev/sda1 * 11 19166 7757824 83 Linux
this tell me that I have only one drive called sda and only one partition called 1
a ATA drive would show up as hda for first drive, hdb for second, hdc for third...
a SCSI or SATA drive would show up as sda for first drive, sdb for second, sdc for third...
partitions are merely numbers 1, 2, 3...
wiping the disk I write
this wil first write "random" data to the entire drive bit by bit, in the process wrecking any partitiontable, filesystem... then it will write zero's to entire drive bit by bit (yeah, will take a while).
dd if=/dev/urandom of=/dev/sda&&dd if=/dev/zero of=/dev/sda
if you just wanted data to be fairly well destroyed so you could start anew, here you go.
if not enough, now for second step.
make disk one large partition and encrypt entire drive, that is a bit more than I can write here, but here you have one link (search web for more, there is a lot out there).
Actually you could likely do it using RSA/SHA like if it was a textfile or E-mail.
use sha or rsa on /dev/XYZ as if it was a file
if you want a "deeper wipe" repeat ALL steps above 25 times/disk
if you want data 100% gone you are out of luck as to simpel computer code, according to authorities only way to be 100% certain is if you melt them down and almost have to stir the thing according to some (not likely needed imho as the magnetic properties will be gone past curie point)