It seems that existing guides of how to boot from USB are out of date, and the one here (https://learn.adafruit.com/external-drive-as-raspberry-pi-root/hooking-up-the-drive-and-copying-slash) didn't work, as it didn't alter the root, though it copied all my files to the SDD.
I don't want to mess up any important files, as I don't know much about them. Is there any up-to-date tutorial for a beginner?