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Until recently, I have successfully installed macOS Monterey 12 dev beta updates on my 2013 Mac Pro (the cylinder Mac Pro) with an Apple 27" LED Cinema Display. The Mac Pro is partitioned into two volumes, one of which boots Big Sur. The other volume is intended to boot Monterey, and it did so successfully through Monterey dev beta 7. However, the dev beta 8 and 9 updates cause the screen to go black near the end of the update process, when I need to be able to start clicking on various buttons to finish setting it up. Hitting the Return key just causes the Mac to beep. I have been unable to find any way to see the screen at this point in the process, despite many attempts. Substituting a different monitor yielded the same result. Has anybody else seen this? Can anybody suggest a solution?

I know my Mac Mini DTK boots because I hear it ask if I want to turn on VoiceOver. But my monitor never displays it. The monitor is an LG UltraFine 24" (24MD4KL-B), which the DTK Release Notes specifically say is supported at 4K using DisplayPort alt-mode. But I have connected it both with an Apple USB-C cable and with a Thunderbolt-2 (Mini DisplayPort) cable and USB-C/Thunderbolt 2 adaptor, both of which should support DisplayPort alt-mode, but I get no video -- the monitor remains dark. My old Apple LED Cinema Display 27" doesn't work, either. Any suggestions?