Do not upgrade your Fusion Drive Mac to Big Sur. After the Mac reboots, the Fusion Drive falls into two parts (SSD and HDD), and the upgrade fails. At that point, you have to rebuild your Fusion Drive from the recovery mode using Terminal and command tools. Then, you have to install Big Sur from scratch and restore all of your Applications, Files, and Settings from a Time Machine backup.
Just finished talking to Apple Support.
- The guy didn't know anything about the problem with upgrading Fusion-Drive Macs to Big Sur.
- When I asked him to look it up, he found multiple reports of other users reporting this problem.
- When I asked him if he would recommend that I hold off on the upgrade of a 2014 5K iMac with Fusion Drive to Big Sur, he said that he would indeed recommend that I hold off.
- When I asked what he is going to do with my call, he said he would create an internal case and send it to the developers.
- When I asked if he thought Apple would fix this problem with the next dot update to Big Sur, he said it was very likely they would.
- When I asked if there is a way to tell whether or not the next dot update to Big Sur will include a fix for the Fusion-Drive Macs if Apple doesn't specifically mention it in the Release Notes, he said I should wait for the next dot update and call Apple Support to see if they can confirm that the release includes a fix for this problem.
At this point, I would strongly advise anyone not to upgrade to Big Sur and wait for a fix - that is unless you are willing to rebuild Fusion Drive and install Big Sur from scratch and then restore all the applications, files, and settings from a Time Machine backup. But even to those who are so adventurous, I would still advise against doing it yet because there are other reports on the web about older Macs not being able to install Big Sur from scratch and getting bricked (mostly 2013 and 2014 MacBook Pros, but who knows if this affects iMacs and Mac Minis as well?.