Catalina reorganized the Mac boot drive. When Catalina is installed the boot drive is converted from one logical volume (HFS or APFS) to a collection of 2 APFS logical volumes, a read-only volume with the same name as the original boot drive and a read-write volume name ‘[boot drive] - Data’. Note that you only see this structure when running Disk Utility or other disk maintenance programs; in the Finder, there is only the single drive icon as usual.
This week, I migrated my old iMac to a new iMac. New machines come with the boot drive name ‘Macintosh HD’, but I always change it to reflect the actual machine. I have always done this by renaming the drive in the Finder by clicking the drive name and typing a new name, in this case, iMac 5K Disk.
Everything ran fine until I fired up Carbon Copy Cloner for the first time. When I tried to run a cloning task, I was told ’ The source volume was not available when CCC attempted to run this backup task’. I checked the ‘iMac 5K Disk’ was identified as the source volume for the task and got the same error. I opened Disk Utility and noticed that the Data volume was still named ‘Macintosh HD - Data’. So, I needed to rename the Data volume via Disk Utility as ‘iMac 5K Disk - Data’. Once I did that, I was able to create a proper clone of the boot drive.