I’ve found it critically important to unplug all peripherals that aren’t needed. I’ve been in reboot loops that never stop! I have yanked the USB cables as the machine shuts down and gotten back to progress. Now I always unplug the unneeded external drives before starting upgrades. I’m writing this as I stare at a white Apple logo going from Mojave directly to Big Sur on my Late 2013 Mac Pro, wondering if it is hung…

I wish I saw this thread before trusting the upgrade process. I'm stuck at ~40%... PRam didn't help. Going to look into the mdworker solution from here: https://mrmacintosh.com/macos-upgrade-to-big-sur-failed-stuck-progress-bar-fix-prevention/ Going from Mojave to Big Sur. Mac Pro late 2013. Should have gone to Catalina first I guess. I wonder if I can recover. I've got different partitions and volumes and USBs that are bootable into Mojave, hopefully this can get be back to a functional state without a clean install.