if anyone is interested, here is what I usually do when I want to wipe a disk
method 1:
http://sourceforge.net/projects/dban/files
like Daiwa already mentioned before
method 2a:
in case you are not using linux boot from a debian live image (https://www.debian.org/CD/live/)
open a terminal
$ sudo fdisk -l # search for the disk you want to delete and remeber its device name (something like /dev/sda)
$ sudo dd if=/dev/urandom of=/dev/sda bs=4M # this will completely overwrite disk a, including boot record etc. with random data, so doublecheck what you write after "of="
you can also wipe partitions, e. g. "/dev/sda2" will be partition 2 on disk a
method 2b:
in case you are not using linux boot from a debian live image (https://www.debian.org/CD/live/)
open a terminal
$ sudo apt-get install cryptsetup gddrescue # install cryptsetup and ddrescue
$ devicel=/dev/sda # write the device you want to wipe into a variable so you can copy/paste the next line
$ sudo cryptsetup open --type plain --key-file /dev/urandom ${devicel} to_be_wiped # create a plain encrypted container using random passphrase
$ sudo ddrescue -f /dev/zero /dev/mapper/to_be_wiped # overwrite container with zeroes, which will be random data on the device
here you basically use the linux encryption software cryptstetup to write random data to your device
using ddrescue instead of dd will give you a simple status feedback
this will be much faster than method 2b and also method 1, about 2-3 hours for a conventional 1 TB hdd (depends on your hardware)
dont be afraid to try out method 2, linux is great for those kind of things