My iMac and my wife's MBP upgrades to Catalina went smoothly. The mini greeted me with a "update failed" message. Then I noticed that the Software Update Pref claimed that 'updates were being maintained off-site'. Never found a workable solution to that and decided to press ahead with getting the OS upgrade completed first.
Tried running the upgrade again and was greeted by the installer showing all drives with the triangular
⚠︎ warning.
I decided the boot drive, at least parts of the OS, might be corrupted, even though it seemed to boot and apps ran fine. Downloaded another copy of the Catalina installer and it succeeded. Followed that with the Security Update and things are back to normal.
In fact, it's possible I may have accidentally run the Catalina
Security updater before the actual
OS upgrade! I wouldn't think that would be possible. OTOH, if the
Big Sur macOS
11 installer didn't check that enough space would be available, the
Catalina macOS 10.
15 Security Updater might not have even checked what the OS
is!