Okay, I gave R1 beta 4 another go and now found that open core lets it partially boot. If I boot from the Mac native boot picker it hangs as soon as the boot splash appears. If I use open cores boot picker it boots all the way till about when the desktop should start and I get a KDL. It looks like this / anyone seen this before?
It seems to be the font manager??
Exact same stick boots on my surface 2 pro fine