I apologize for the length of this post; I wrote it for a number of folks (pity them) who rely on me to keep them advised about things such as is it safe to update to Big Sur 11.3.
I’ll start by directing you to this site, a great site for Mac users:
https://eclecticlight.co
Then direct you to a page at that site regarding problems associated with updating to Big Sur:
https://eclecticlight.co/2021/04/28/that-macos-update-has-caused-havoc-what-do-i-do-next/
Then note this exchange in the comments appended to that page. The question is being asked by someone who installed the latest update to Big Sur and wanted to reverse his decision:
Q. What about regular TM Backups/Snapshots, can’t someone revert to them?
A. Yes you can, if TM is making them. In Mojave, you’re probably in luck; in Catalina, you might not be.
In Big Sur, AFAIK it’s not possible to back up or make snapshots of the System volume. Also remember that TM automatically deletes snapshots once they’re 24 hours old.
This I did not know.
And now I have to admit that I am in unchartered waters and not sure of my facts but it appears to me that if you are running Big Sur, updated to the latest version of Big Sur, are using Time Machine for your backups, left Time Machine attached, had a problem with your update
and that problem was transmitted to your Time Machine Backup, you cannot designate a specific Time Machine snapshot from which to revert. Your TM machine
may be hosed/corrupted/whatever.
My new rule for future updates will be:
eject and shut off Time Machine and any other backups when I choose to update my macOS.
On my 2015 iMac, my Big Sur update produced something unique and unusual: It placed 70 folders, 29 of them empty and the remainder completely full of sub-folders and files I have created over the years. Some of these files and folder came from my Home folder, and some came from my iCloud Drive folder. It appears that everything the update placed in the Trash still exists on my macintoshHD, so it looks like I am OK. But I am not in any hurry to empty the Trash.
When I posted my problem to the site I mentioned at the outset of this email, the response was:
I wonder if your boot disk had an APFS problem on it.
And there you have my saga. Good luck with your update & may it be trouble-free.