Você tem a ordem errada. Assumindo sdb1 ...
sudo dd if=/dev/urandom of=/dev/sdb1
sudo cryptsetup -y -v luksFormat /dev/sdb1
sudo cryptsetup luksOpen /dev/sdb1 $name #change "$name" to any name you wish
# now format
sudo mkfs.ext4 /dev/mapper/$name
sudo mkdir /media/mount_point
sudo mount /dev/mapper/$name /media/mount_point
Veja:
link