Moved my #raspi root partition to my external TB drive:

1. TB: Shrink 'data' partition
2. TB: Create new 'root' partition
3. rsync raspi "/" to TB's new partiton
4. rpi: Tell /boot/cmdline.txt where the new root partition is
5. rpi: Add "/" entry to /etc/fstab
6. rpi: Reboot

That was fun :)