Mr Macintosh reports a problem with the installer for Mac OS 11 Big Sur 11.2 (and possibly 11.1). If you try to update your computer from Catalina, Mojave or High Sierra to Big Sur 11.2 and have less than 35.5 GB of free space, but more than 13 GB, you may encounter a bug that causes the system to try to update but fails and crashes. In the worst case, you may be forced to reformat and restore from backup.
The problem is that the installer does not correctly check that there is enough free space before starting the update.
If you have Filevault activated, you can also not start to Reset because you have to fill in an admin password and it does not work properly when the hard disk is completely full.
The solution that Mr Macintosh has found for affected users is to use Target Disk Mode and connect the locked Mac to another Mac to clear some files and free up space. If Filevault is enabled, the other Mac must run Mac OS 10.13 High Sierra or 10.14 Mojave – it does not work with Catalina or Big Sur.
If you already have Big Sur 11.0.1 or 11.1, you are safe. If the update fails, the system is automatically rolled back.