This also means I have an unlocked bootloader but not S-Off.
I flashed the latest build of TWRP over stock recovery using fastboot, and flashed the Superuser zip through it. Although super user is installed and root access is enabled, every time I try to write to system (e.g. installing Busybox) the phone just crashes and reboots. Changes made don't seem to stick either. Same results with a Superuser + Busybox zip flash.
Do I need S-Off or is this a one off problem?
HTC One, 4.4.2, completely stock with TWRP