This tutorial describes how to perform full disk encryption, partitionless, on a external block device using Cryptsetup.
apt-get install cryptsetup libblockdev-crypto2
From Cryptsetup's FAQ:
If the target was in use previously, it is a good idea to wipe it before creating the LUKS container in order to remove any trace of old file systems and data. For example, some users have managed to run e2fsck on a partition containing a LUKS container, possibly because of residual ext2 superblocks from an earlier use. This can do arbitrary damage up to complete and permanent loss of all data in the LUKS container.
cryptsetup open --type plain -d /dev/urandom /dev/sdX to_be_wiped
dd if=/dev/zero of=/dev/mapper/to_be_wiped status=progress
cryptsetup close to_be_wiped
cryptsetup luksFormat /dev/sdX
cryptsetup open /dev/sdX to_be_formated
mkfs.ext4 /dev/mapper/to_be_formated
cryptsetup close to_be_formated
Mount the drive:
cryptsetup open /dev/sdX crypt
mount /dev/mapper/crypt /mnt
Unmount:
umount /mnt
cryptsetup close crypt