LUKS

Full disk encryption is stupidly simple with the cryptsetup package.

These commands can be used to take a disk, encrypt it, and create one or more file systems underneath it:

cryptsetup luksFormat /dev/???
cryptsetup luksOpen /dev/??? <name>
mkfs.btrfs -L <label> /dev/mapper/<name>
cryptsetup luksClose <name>

Simply substitute ??? with the drive, and <name> with whatever you want to call it.

You can use any file system; I happen to prefer btrfs.