ubuntu@ubuntu:~$ sudo dd if=/dev/urandom of=/dev/sda6

(12+ hours elapse)

dd: writing to ‘/dev/sda6’: No space left on device
660092929+0 records in
660092928+0 records out
337967579136 bytes (338 GB) copied, 39571.4 s, 8.5 MB/s
ubuntu@ubuntu:~$ modprobe

ubuntu@ubuntu:~$ modprobe dm-crypt
ubuntu@ubuntu:~$ modprobe aes-x86_64
ubuntu@ubuntu:~$ sudo modprobe sha256
ubuntu@ubuntu:~$ cryptsetup luksFormat /dev/sda6

WARNING!
========
This will overwrite data on /dev/sda6 irrevocably.

Are you sure? (Type uppercase yes): YES
Device /dev/sda6 doesn't exist or access denied.