MacOS Catalina 10.15.7 Supplemental Update 2 causes circle with a line through it
I use a Mac Pro with an external SSD. Both the internal drive and the external drive have Catalina 10.15.7 installed and I boot from either depending on what software I want to run.
The second time the computer started up after installing the Supplemental Update 2 on both drives, the screen went to the "circle with a line through it" icon. This was after entering a user password at login. The first time, a restart fixed it. Now it appears to be a constant problem booting from either drive.
What I see:
Computer starts and reaches the user login screen.
After entering the password I get an apple with progress bar screen.
Then the circle with a line through it screen.
Fixes attempted:
Repeatedly restarting - No longer helps.
Safe mode (shift key held down at startup) - Boots fine in safe mode for the internal drive. (I cannot seem to find a way to do a safe boot for the external drive.)
Recovery mode (Command R) - Disk Utility found no problems with either drive or any of the volumes. (Followed instructions here: https://support.apple.com/en-us/HT210898)
PRAM reset (Option-Command-P-R) - Sometimes works on the next boot but only for the internal drive.
Clear Caches (~/Library/Caches/ and /Library/Caches/ on both drives) - No effect
Reinstall the OS - It is an overnight download on this internet connection but that is next.
Am I missing anything obvious?
Mac Pro, macOS 10.15