How to setup a second encrypted hard drive and move your home directory to it:
Let's make a keyfile for it second drive, which I'll be calling cryptextra.
Restrict access to root
sudo dd if=/dev/urandom of=/etc/keys/cryptextra bs=2048 count=1
Setup LUKS on second drive. Be absolutely sure you have the right drive! If you get it wrong there is NO undo!
sudo chmod 600 /etc/keys/cryptextra
Open cryptextra and make a filesystem.
sudo cryptsetup luksFormat /dev/sdb1 /etc/keys/cryptextra
Time to setup some config files to auto mount. First, put an appropriate entry in /etc/fstab
sudo cryptsetup --key-file /etc/keys/cryptextra /dev/sdb1 cryptextra
sudo mkfs.ext4 /dev/mapper/cryptextra
And /etc/crypttab. 'noearly' is optional, but you will get a prompt to skip mounting or recover manually on boot if you don't have it.
UUID=abcdef.... /mnt/cryptextra ext4 defaults 0 2
Use 'blkid' to get the UUIDs above. IMPORTANT: The LUKS device UUID goes in crypttab, the UUID for the actual filesystem goes in fstab!
cryptextra UUID=123456 /etc/keys/cryptextra luks,noearly
You should reboot at this point to make sure everything works correctly up to this point.
It's time to start moving our home directory.
I had trouble with the next part. Supposedly, all you have to move is the .ecryptfs and .Private folder. That didn't work for me, I had to move the files separately of them. Copy everything in your home directory besides to two ecryptfs files somewhere safe.
sudo mkdir -p /mnt/cryptextra/home/user
sudo chown user:user /mnt/cryptextra/home/user
Change your home directory in /etc/passwd. The file is quite simple and this should be obvious.
rsync -avP .ecryptfs .Private /mnt/cryptextra/home/user
Log out and log back in.
If you want to permanently set the home directory for all new users you create later, edit /etc/default/useradd.