Mike's response to my question may be "quoted" as "Installing Big Sur on top of a CCC data clone and running Migration Assistant on top of a Big Sur installation yield the same results."
Agreed. But that does not mean you can clone or re-clone a data volume onto an existing Big Sur drive and end up with a bootable system.
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artie: Can I install Big Sur on an external drive and keep it current by cloning my internal data volume to it regularly?
Mike Bombich: CCC will keep the Data-only volume current, yes. Nothing copies the System volume – not CCC, not Time Machine, nothing nor nobody can copy it (yet), not even Apple. The only way to restore the system right now is to reinstall it. FWIW, I'd wait on this one. There is no impetus to upgrade to Big Sur, and there are some lower-level things that are still not working correctly. The biggest problem right now is that the "Signed System Volume" is supposed to be cryptographically "sealed", but that seal is constantly broken. A volume with a broken seal isn't supposed to be bootable, but Apple has relaxed that requirement because the functionality doesn't work. You're in no worse shape than in Catalina where the volume lacks the seal altogether, but then there's also no advantage (security-wise) to upgrading just yet.
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artie: Will the external Big Sur installation be bootable after its Data-only volume has been updated by CCC?
Mike: That is my expectation, yes (it has remained bootable in my own tests).
It may be worth both your whiles for you to report your reverse prohibited symbol experience to Mike. (Broken/UNbroken seal issue, perhaps?)
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In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
