IMac won't boot 6mths after Catalina upgrade - 2 HD volumes in one container

[Luka374]

With Catalina already installed months ago, worked fin until the computer suddenly shows an Os install error, and won't boot. Recommends to shut down and start Diagnostics mode which is won't do.

• Won't boot in Safe mode (maybe due to Filevault).
• Tried Reinstall from Recovery mode - downloads then shows same error
• Fails to repair disk when running First Aid - "Can't repair as other APFS volumes in its container are mounted"
• In Startup Disk - no options given

In the Disk Utility, I can see two volumes on HD -> Container 1 -> Mac HD and Mac HD - Data (I dont think one wasn't there before).

Is there a way to fix this without losing data. I live in Croatia without an AppleStore in easy reach.

Many Thanks

 

[nambuccaheadsau]

Do you have a current backup and was Catalina Installer put on an +8GB thumb drive?

If so boot from the Installer and run Rep[airDisk. If no joy a format and a clean install and use Migration Assistant to copy over from backup.