@Chris-Stratton I'm not sure it is power but not really sure how I can test that? Currently it is drawing the power and serial via my MacBook Pro's USB port, if I power via a PSU I lose my serial connection to trigger the upgrade at all.
I did also try ensuring only the upgrade file was on the stick, it's very odd that the 4GB isn't working but the 128GB is. I am going to try and find a major brand smaller stick, Sandisk or whatever, just in case the stick itself is oddly configured/designed.
Any suggestions for how to test things are appreciated!