In the past, the Startup Disk Preference Pane would list all available startup volumes, including volumes cloned by CCC (whether CCC used ASR or its own file copier). Some Big Sur cloned volumes do not appear in the Startup Disk Preference Pane, despite being perfectly bootable.
We have reported this issue to Apple (FB8889774) and we are currently awaiting a response.
Workaround: To boot from the cloned volume, restart your Mac while holding down the Option key, then select the cloned volume in the Startup Manager. When your Mac has completed booting, you can optionally choose to set the startup disk to the current startup volume (i.e. if you want the Mac to always boot from the cloned volume).
Starting in macOS Big Sur, the system now resides on a cryptographically sealed "Signed System Volume". That volume can only be copied using Apple's proprietary APFS replication utility ("ASR"). Right now, ASR will only copy whole volume groups (System and Data), we can't choose to clone just the System volume. As a result, every time an OS update is applied to the source, we would have to erase the whole destination volume (including any existing snapshots on that volume) just to update the system on the destination.
To avoid deleting your snapshots and the rest of your backup, CCC will not update the System volume on the destination when System updates are applied to the source.
We made a feature request to Apple in September 2019 (FB7328230) to allow ASR to clone just the System volume. Apple's APFS team acknowledged the request in June 2020 and clarified the requirements, and now we're waiting on the implementation.
Our recommendation: We recommend erasing the destination only for the purpose of establishing the initial bootable backup. CCC can then use its own file copier to maintain the backup of your user data, applications, and system settings. If you would like to update the OS on the backup volume, you can boot your Mac from the backup and apply any updates via the Software Update preference pane in the System Preferences application. This is not something that we anticipate you would need to do frequently, nor even proactively. You could apply updates before attempting to restore from the backup, for example, if that need ever arises.
In the current shipping version of macOS Big Sur (11.0.1), Apple's ASR utility cannot replicate the startup disk in an M1-based Mac. Attempting to do so results in an error:
'Apple System Restore Tool': Source volume format not yet supported in this version of macOS
Based on that error string we can conclude that Apple is aware of the problem and is presumably working towards resolving it for a future update to macOS. CCC 5.1.23 will automatically perform Data Volume backups on M1 Macs and avoid any attempts to copy a System volume on those Macs. When Apple posts an update to macOS that resolves the ASR problem, we'll post an update to CCC that adds back support for copying the System volume on these Macs.
Finder will show and allow you to customize the volume icon for your current startup disk, but not for other Catalina- or Big Sur-bearing startup disks that your Mac is not currently booted from. This problem is not specific to CCC backups, but we see this frequently because CCC is creates bootable backups. This problem is the result of a design flaw in the implementation of custom icons in an APFS volume group. Up to macOS Catalina, the custom volume icon is stored in a file at the root of the startup disk named ".VolumeIcon.icns". To keep the System volume read-only, yet allow the apparent modification of this icon file, Apple chose to create a symbolic link at the root of the startup disk that points System/Volumes/Data/.VolumeIcon.icns. For the current startup disk, this path resolves correctly because the Data member of the volume group is mounted at /System/Volumes/Data. That's not the case for external volumes, those Data volumes are mounted at /Volumes/CCC Backup - Data (for example). As a result, the symbolic link to .VolumeIcon.icns is unresolvable for any volume that is not the current startup disk.
We have reported this issue to Apple (FB7697349) and we are currently awaiting a response.
Finder will let you rename the current startup disk, but you won't be able to rename any other startup disks that have an installation of Catalina or Big Sur because the System volume is mounted read-only.
Solution: Unmount and remount the volume in Disk Utility, then you will be allowed to change the volume name.
We have reported this issue to Apple (FB8912480) and we are currently awaiting a response.