When I upgraded to Catalina, the program apparently created a new Photos file it called "Photos Library.photoslibrary" and made it my System Photo Library. Meanwhile, my pre-catalina library which I had named "Master Photos Library (iMac)" was still there in my Pictures file in finder but all photos loaded subsequently to the upgrade were now placed in the upgrade manufactured file "Photos Library.
Once I noticed this, my concern was using all that memory (both libraries were about 145GB) AND which one to use AND how to be sure I didn't lose the photos taken since the upgrade.
I called tech support and since I was using iCloud photo, the solution was relatively easy, though time consuming. What she sumised had happened was that when Catalina and Photos 5 tried to convert my library, it was very disappointed to not find the file Photos Library.photoslibrary so it created one leaving my self named file intact.
Why not just keep the newly created one with the recent photos? Because, she explained (and demonstrated) that my existing "projects" would not have copied into the new Photos 5 under Catalina! So...
We first opened the iCloud website, went to Photos to assure ourselves that all 43,000 photos and videos were in the cloud. Then we opened Photos and turned off iCloud photos. Then we closed Photos then reopened it while holding the "Option Key" bringing up a box allowing us to designate my OLD file as the system library then we re-opened Photos, turned iClound on and waited, and waited, and waited till (at the very bottom of the all photos page) we could see that it was beginning to sync with i cloud. This will bring my old library (named Master Photo Library iMac.photoslibrary current with not only my existing project but also with my most recent photos. I set up a return call with support for the morning and went to bed.
Lo and behold, Photos 5 had completed its work overnight and I now had a fully functioning Photos 5 under Catalina BUT I still had two photos libraries.
Once I brought the new tech support rep up to speed, we opened Finder and could see both libraries in Pictures and knowing now that the Catalina created Photos Library.photoslibrary was now an unused orphan, we dragged it to the trash (NOT deleted, yet). Now we are left with only one library to be accessed by Photos. We opened Photos and made sure my projects were there AND my most recent photos and that the number was about right (some 43,000 give of take). Then, to avoid confusing the next upgrade gremlins, we shut down Photos, went into finder and changed the name of the now only photos library in Pictures FROM my "Master Photos Library iMac. photoslibrary to the generic and apparently preferred filename of Photos Library.photoslibrary.! QED!
I am now putting Photos 5.0 though its paces and making sure everything works, is there, etc. and only after a few weeks will I have the guts to empty the trash and recapture over 140GB of storage.
Long winded but I hope this will help someone out there with similar issues with the Catalina and Photos upgrade.
Don
