If you don't have a bootable backup of 12.1, let me walk through the following:
- When you started the upgrade from 12.1 to 12.3.1, the system downloaded the update and began to prepare the update.
- Then macOS rebooted. This is Reboot #1.
- When OpenCore boot picker appeared, a new boot option appeared and it was selected by default. Do you remember that?
- This boot option was called Macintosh HD or something similar. This is a temporary name given by the macOS installer.
- When this boot option starts, an Apple logo will appear on a blank screen. A horizontal progress bar will appear under the Apple logo. This is the well known progress bar.
- About 1 minute later the progress bar should reach the end and the system should reboot. This is Reboot #2.
- Back at the OpenCore boot picker we should see the default boot option to be either Macintosh HD once again or the real name of the boot disk.
- The Apple logo and progress bar will appear again. We might see About 10 minutes remaining... or the system might reboot one more time before we get to About 10 minutes remaining.
At which point are you currently stuck at?
I just nuked my SSD with the Monterey install on it and plugged in my Big Sur USB installer.
Ran through the full install and setup, mounted my EFI partition and put the 0.8.0 EFI I shared with you earlier and rebooted and it was working fine. I rebooted several times and during the Open Canopy boot picker it shows in the bottom right the version text and says 0.8.0. So Big Sur appears to be fine.
So I proceeded with downloading the Monterey 12.3.1 update again and went through the installation again. I didnt note down the name at point 3 but it proceeded as you said and at point 8 it said 23 minutes remaining. It then rebooted again and said 10 minutes remaining and then rebooted again to the default option, but as soon as I select it, it has the same issue of going a quarter of the way through on the progress bar and then rebooting back to bios and the boot picker.
I read somewhere about the AppleMCEReporterDisabler kext so I tried enabling that and I managed to boot into Monterey 12.3.1! But then it came up with loads of WindowsServer errors and the monitor just went black.
Thanks for your support Casey.