To answer my own question, I found this on the bombich.com website:
" Other Catalina and Big Sur startup disks can’t be renamed in the Finder
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 right-click on the volume in Disk Utility’s sidebar and choose the option to rename the volume.
We have reported this issue to Apple (FB8912480) and we are currently awaiting a response."
My external drives are copies from my internal one using SuperDuper, not Carbon Copy Cloner, but the principle is the same, although the suggested solution doesn’t work for me. (The one I mentioned above in my original post does, at least as a temporary workaround.)