Alright, time for some status updates, specifically interesting for
@CaseySJ and
@Mightymouseuk007 who originally reported an EFIClone issue.
Today I acquired a 970 Pro which I'm using as my test drive for Big Sur. I'm using the complete drive, newly formatted, to ensure that no APFS shenanigans lay undetected until later on.
First off, the install itself went swimmingly, just like the first time. This time I did not do an upgrade but a full-fresh install by creating an Install Media USB Stick. I booted from my main drive's EFI, selected the "Install Big Sur" entry and went off to the races. Everything worked fine and boot option selection was automatic, as expected (due to working NVRAM), so a completely unattended install.
The first time around I booted into the setup screen after the main installation was done and tried to migrate all my data from my existing disk. The process itself worked and I was able to access the desktop afterwards. However, after rebooting, I encountered the same issue as before where the boot would fail due to some possibly APFS-related issues.
My thinking was that migrating the data instead of upgrading would simply copy the data to the new APFS structure and hence it should be compliant and boot without issues. This is not the case. Neither a direct upgrade nor a later migration work. The only option is a fresh install without data migration.
In any case, I did eventually wind up with a working system that was capable of rebooting reliably without failing.
I then set out to test
EFIClone_v2.sh to reproduce whatever issue mightmyouse was having. Turns out the script worked perfectly fine on my end, in a dry run. It found the correct EFI partitions of both the source as well as the target disk and initiated the copy. As such there might be an issue on their end instead, or there is an issue in the script in the case that mightymouse has a very unusual disk configuration or something.
If you could,
@CaseySJ, since you have Big Sur up and running as well, could you give it a try? It would be good to get a third data point to see the trend that's forming. You can invoke the script via the command line directly for convenience as well.
I suspect the bootable backup issue may be more related to CCC itself not being updated to fully support Big Sur yet.
This is the note from their latest pre-release:
This means
no bootable backups on Big Sur for now.