I have a MacBook Pro Mid 2012. Until yesterday it had High Sierra. I tried to update to Catalina with an installer I found online, but the installation brakes up every time and starts again, once the computer is restarted. I get the Error: -69845: File system verify or repair failed.
Luckily, I have an older copy of the system on an USB stick, so I was a able to boot the MacBook and access the drive. It is locked, so I can´t write it, but at least I can still backup and save my data.
The question however is, is there something I can do to save the old installation or is it corrupt now, because the update to Catalina overrode some files and there is just nothing I can do to fix that?