Made a bit of an oopsie with the rebuilt backup drive. Both Time Machine & CarbonCopyCloner completed their backup duties but then I noticed I's swapped partitions, the clone of the 512 GB SSD was on the 1.5 TB partition & the Time Machine backup was on the 500 GB section, not what I had intended. I try to have Time Machine backups of at least 2.5 times the size of the boot drive.
Back to Disk Utility to erase the drive. It failed, the two partitions were unmounted & wouldn't come back up, no matter what I tried.
So I switched the drive to the MacBook Pro & erased it & redid the partitions as I should have done in the first place, hey, it was 4:30 am & I should have been sleeping.
Ejected the HD & reattached it to the MacBook Air. Set up backup apps to the correct partitions. Backups commenced on correct partitions.