@Humpleupagus It doesn't work with GNU mv, as that selects multiple directories and; mv: target '/dev/null': Not a directory

I think you meant: mv / /dev/null and well; mv: cannot overwrite non-directory '/dev/null' with directory '/'

shred -n 0 -z /dev/sda is what you want to erase a drive completely (you could overwrite with random data, but that's pointless really).