LMS dead after backup, update etc.

> necessary. It seems to me that it is connected to the right (or better
> lost rights) of LMS to read HDD files. I am not an expert in Mac OS but
> probably someone has an idea what could cause this and how to stop this
> annoying behaviour...

You'll indeed have to grant LMS permission to access external disks.
It's a macOS "limitation". There are plenty of references in this forum.