Given the proprietary nature of Apple's new Signed System Volume, it's no longer possible to copy the macOS System volume using conventional system calls and copy utilities. We coped with this scenario in the macOS 10.15.5 update by using Apple's proprietary APFS replication utility to clone the System volume. Unfortunately that does not currently work on macOS Big Sur, ASR bails immediately claiming that the System volume is not sealed. Indeed, we have confirmed in Big Sur Beta 5 and Beta 6 that a fresh install of macOS produces a System volume with a broken seal (and the problem persists in Beta 7). We are very hopeful that Apple will defer release of macOS until this bug is resolved.
We have reported this issue to Apple (FB8488112) and we are currently awaiting a response.
Workaround: To make your backup disk bootable, install macOS Big Sur directly onto the backup disk.
All Big Sur startup volumes are mislabeled as "EFI Boot" in the Startup Manager (the screen that appears when you hold down the Option key on startup). This mislabeling can make it difficult to distinguish two installations of Big Sur on the same disk, or two installations of Big Sur on multiple external disks.
We have reported this issue to Apple (FB8625219) and we are currently awaiting a response.
Workaround: If it is impossible for you to distinguish multiple EFI Boot volumes in the Startup Manager, you can change the startup disk in the Startup Disk Preference Pane in the System Preferences application instead. If you are unable to boot from your production startup disk, hold down Command+R to boot into Recovery Mode, then you can choose "Startup Disk" from the Apple menu.