Mjoave refusing to boot from CCC backup (High Sierra)

[nemofish]

I have completed a clean install of Mojave on my 2012 Mac Mini; before doing this in order to wipe my SSD I made a bootable backup in Carbon Copy Cloner, this worked fine.

However now Mojave is installed, I cannot boot into the Sierra backup. I see it in startup disk options but if I select this it just ignores it and boots into my new Mojave install.

If I restart holding down the Option key, this volume doesn't appear.

If I restart using OSX recovery, it again shows the drive as a startup disk but when selecting this option it again just boots into my SSD running Mojave.

 

[Fishrrman]

I make no promises or guarantees, but try running this AppleScript on the CCC backup:

 

[treekram]

What the System Preferences -> Startup Disk uses to look for candidate startup drives is different than what is used when you press the option key at start.

I'm guessing that because the System Preferences does recognize your disk as the startup disk, the blessing is not an issue.

To me, it appears that there's some issue with your particular external disk. The Startup Manager which is invoked by pressing the option key at start is contained in the Boot ROM of the Mini and it has more stringent criteria for what it will recognize as bootable. Take a look at the following link:
https://bombich.com/kb/ccc5/what-makes-volume-bootable

Have you been able to boot from the external disk before? Do you have an integrated external disk or a docking station/enclosure? As mentioned in the link above, there could be a problem booting from these. If you have an older docking station, these probably are the most problematical. If you have an enclosure, do a search for the model # and "macos boot" and see if other people have problems. Also, using the Disk Utility app, make sure the disk has the "GUID Partition Map", not the "Master Boot Record". You have to select the disk to see this, make sure the View->Show All Devices is selected.

I have the 2012 Mini with Boot ROM Version 276.0.0.0 and it doesn't have a problem finding Sierra boot disks as startup so I don't think there is an issue with this Boot ROM version which you should have if you just recently installed the latest version of Mojave. You can check what Boot ROM version you have in the System Information app. It will appear in the first panel that comes up - "Hardware Overview".

 

[jbarley]

I have completed a clean install of Mojave on my 2012 Mac Mini; before doing this in order to wipe my SSD I made a bootable backup in Carbon Copy Cloner, this worked fine.

By "this work fine" you mean you tested the backup and it actually booted?