KB5077181 was released about a month ago as part of the February Patch Tuesday rollout. When the update first arrived, users reported a wide range of problems, including boot loops, login errors, and installation issues.
Microsoft has now acknowledged another problem linked to the same update. Some affected users see the message “C:\ is not accessible – Access denied” when trying to open the system drive.
I dunno? It sounds very plausible, exactly the kind of thing that Windows would do. I posted about it to Metafilter some time back and no one there seemed to think it couldn’t happen.