I play around with my RPI a lot, but always end up with /root on an SSD.
super performance!
 

ROOT on an SSD rocks.

sudo mkfs.ext4 /dev/sda1 -L root
sudo mkdir /media/newdrive
sudo mount /dev/sda1 /media/newdrive
sudo rsync -avx --progress / /media/newdrive
sudo nano /boot/cmdline.txt

Then into the cmdline.txt put

root=/dev_/sda1 rootfs_type=ext4 root_wait

(remove the _’s before pasting.)

Doesn’t really matter if raspbian (what i’m using now) is using PARTUID, just replace it for /dev/sdaX