Big Sur Update
If anyone else is having the issue where they are upgrading to Big Sur but they get an update error involving the Startup Disk, here is what I did to fix it.
After clicking “OK” on the error and you see the Startup Disk selection window that never populates anything, right behind that window is the Update Window (cannot for the life of me remember exactly what this was called). Click on it to make it active and then click on the menu header beside the Apple icon. Go to the bottom of the menu and click quit update utility. Another window pops up and I clicked restart. After several repeats of the Apple loading screen (and I do mean several and they can take a long time), the Mac finally reboots properly and updated. This worked on the 2018 MacBook Air I am using.
MacBook Air
