I cannot get my MacPro 3.1 early 2008 to recognise a bootable USB stick. I'm trying to upgrade from 10.11 El Capitan, to a newer version using the unoffical patcher tools. I've tried both Mojave, and High Sierra and niether will work.
The USB stick is formatted Mac OS Extended (Journaled), with a GUID partition map.
I have verified that the stick is working using my Macbook Air. The Air recognised and booted fine from the USB stick when trying both macOS versions, so the stick is fine.
I've tried holding the Comand + Option + R + P at boot multiple times, which hasn't worked.
I've also tried multiple USB ports, including one that a timemachine external drive was connected to (which does interesting show on the boot options screen) and still nothing.
Why is this not working? Does anything need to be enabled?