I am having the same problems. (just posted same question)
I do know for me my ext HD (Time Machine) did work. What happens with me is I connect the HD, systems crashes, then I don't do anything and let the computer reboot. For me the drive appears and I can let it resume its backup process.
I just downgraded back to 15.6 and wait to see if Apple corrects the problem.
stevenash2001
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