Do you get a question about upgrading the catalog each time you open Lightroom? If so, the explanation is probably as follows: in your Lightroom preferences the 'MO L R' catalog is set as the catalog to open, but this is your older pre-upgrade version. So each time you open Lightroom it upgrades this catalog and makes a copy. The first copy was called 'MO L R-2', the next one 'MO L R-3', etc.
Thanks for your help.
1. what is the difference between Mo's LR-6 and Mo's LR-6 Previews.lrdata,
2. I would like to deep just one, so which one is safe to get rid of it?
Thank you so much,
You need both. The one is your catalog, and the other is your previews folder. If you delete it Lightroom will create another one the next time you start Lightroom.
OK now why the file for MO'LR is bigger than the rest?
I would like to know which is safe to delete and which one is to keep.
Thank you all,
We cannot answer that question for you.
You need to understand what and why you have each catalog file.
If the names are based on the version of LR that the old catalog was upgraded to, you may need only the "6" file as long as it contains all the images that the previous catalogs have.
That would be one more indication that my suggestion is correct. It is an old Lr6 /CC2015 catalog. Lightroom Classic has a new catalog structure, which compresses the edit history. The result is that the same catalog upgraded to Lightroom Classic is considerably smaller.