A bug allows the upgrade from Catalina, Mojave or High Sierra to Big Sur to force the computer to completely reinstall. All that is required is that you have the wrong amount of free storage space on your hard drive.
According to information, the bug is about the installation program for Big Sur not checking the amount of free storage space correctly.
This means that in the worst case – if it goes really wrong – you will need to format your hard drive or restore your computer from a backup.
Reading tips: Apple releases macOS 11.2.1 “Big Sur” – bug fixes only
- This problem occurs when you upgrade to:
Big Sur 11.0, 11.1, 11.2, 11.2.1
- Problems occur when upgrading from:
Catalina, Mojave, High Sierra
The bug is activated when you have between 13GB and 35.5GB of free space on the hard disk. A very specific amount that is worth keeping track of.
An important detail for users with FileVault enabled is that it prevents recovery of the computer. You have to fill in a password and the process does not work properly when the hard disk is full.
If your computer already has Big Sur 11.0.1 or later, you are completely safe from this bug.
The solution for affected users is Target Disk Mode
Anyone affected by the bug has a solution in Target Disk Mode. By connecting the affected computer to an unaffected Mac, you can delete files from the hard disk and free up space so that the installation can work properly.
Note that FileVault can be adjusted a bit again. FileVault requires the unaffected computer to use macOS 10.13 “High Sierra” or 10.14 “Mojave”.
So it is NOT possible to use a computer with either Catalina or Big Sur, which is a bit sour of course. Many people are quick to update their computers, so it can be tricky to find one that works.