dd if=/dev/random of=/dev/sda
That would take ages. You probably meant 'dd if=/dev/urandom of=/dev/sda'.
One more character before "random" that makes much difference on the running
time! That latter command is indeed analog to 'shred -n 1 > /dev/sda'.
'shred' is part of coreutils and, as such, present on virtually any modern
GNU/Linux system, including Live systems.
'shred' is the best solution. It writes random data instead of zeros. By only
writing zeros (as in 'cat /dev/zero > /dev/sda'), expensive equipments can
still recover some data (because writing a zero on top of a zero leads to a
"magnetic state" that is closer to zero than writing zero on top
