Thanks James for the reply.
I tried the advanced exlcusion pattern, but somehow got it wrong (the item was added again). I guess I have to grok the syntax and logic behind those patterns more before I can confidently write them up. A "Test" function would be cool where you would drop an item (file or folder) onto a little field next to a pattern and it would tell you whether that pattern would match the item or not. But again, that's my fault of not yet feeling confident with the syntax and options.
But anyway, yes, the pattern exclusion will be the next thing I'll try.
The item I want to exclude is a folder (and all of its contents), named:
/Volumes/Daten/CaptureOne Fotobibliothek/_LEGACY Christian Aperture Fotos
The above is the path obtained by using Finder's contextual menu item with the "Copy ? as pathname" option. Do I need to prefix this with something else on Catalina? Which pattern type, and which syntax would I need to use to exclude that folder (and its contents)?
Thanks,
Christian