Before downloading the latest OS version, I thought it would be prudent to back up my existing 10.3.1 to an external drive.
So I did, using CCC & having those preferences set to "Make Bootable". This has always worked fine in the past.
Now, although that backed up 10.3.1 shows up as a bootable system, when I select it as my startup, I get the Apple symbol, but it goes no further.
I've repaired permissions, repaired the disk, but nothing helps.