I have a slightly different problem. Finally deciding to upgrade to Big Sur four days ago; just got my computer back from the shop, less $200 in my pocket because Big Sur froze at 40% (lots of people apparently have had this issue). Got my 600gb backup from Time Machine reinstalled. Great. But now the TM won't back up.
I get a message that "there isn't enough space on "Backup" and I need to either buy a new 1T backup or delete files. The message on the TM backup pop up window still says "Older backups will be deleted as needed" blah blah, so does it still do this or not? Do I need to manually delete old Backups because I had to restore from Mojave backups? And if so, what's the hidden secrets to do this correctly?
I go into TM and I see that 55GB has been backed-up, that there is 418GB available, that all the past backups are gone, that there is something just called "data" bu now, and I get a message there isn't enough room. Huh?
Since, I made the assumption I could "just simply" upgrade to Big Sur and, when that didn't work, then I could "just simply" install a fresh Big Sur using Command R, and when that crashed the entire computer (after having erased the hard drive of course), and then had to have the experts step in and do a restore from my Time Machine, what are the special steps that I don't know to ask to save me time, money and more frustration? Thanks.