I upgraded to 12.3 on a 2019 Mac Pro. It no longer recognizes the internal Radeon Vega II GPU, it just says GPU, so the monitor connected to that doesn't get a signal. Luckily I also have an external eGPU, which it still recognizes.
My problem is that Diagnostic Mode, System Recovery, Internet Recovery, designating an external disk for start-up, etc. all require the use of the internal GPU. It doesn't see the external eGPU when I boot in these modes.
I tried resetting the NVRAM and SMC as well while on the phone with apple care.
I'm currently wasting today trying to figure out a way to be able to revert or re-install without any of those modes. I have another internal SSD from OWC arriving tomorrow, but I'm thinking that there won't be a way to install Big Sur, or even 12.2 (which was working fine), without booting in those special modes.
Please tell me I'm wrong... ;-)
I downloaded the Big Sur installer but when I try to run it, it tells me it can't be run from a newer version of macOS.
