My computer will not restart after Monterey update
I have a 2018 iMac. In early 2021 I bought an external drive (G-drive) and I run the OS on that SSD. My computer prompted me to do an update yesterday to upgrade to Monterey. My computer will not restart. The result is the same whether I press and release the power button, or keep it pressed throughout the cycle: I see the Apple logo, then in six languages a notice that says “Your Computer restarted because of a problem. Press a key or wait a few seconds to continue starting up.” Then I see a progress bar, but the startup cycle begins again. After a few cycles I see a circle with a line through it (a prohibitory symbol) and “support.apple.com/Mac/startup before the screen goes dark.
I have a third-party Bluetooth keyboard (my original Apple keyboard is in storage), which might present a problem with trying to start in safe mode. I’d appreciate any advice.
iMac 21.5″ 4K, 12.5