Recover photographs from Photos library I made the (idiotic) decision to install a beta version of OS X Mojave Big Sur, which later became corrupted, forcing me to reinstall OS X Catalina.
During the time that I had Mojave Big Sur installed, I opened Photos which migrate the library to its new format.  My Pictures folder was archived by Backblaze.
After I installed Catalina, I attempted to open the Photos library (restored from Backblaze) and got this message:

I don't recall the process exactly, but I was able to open Photos which included some of the original photos.  As you can see, this new library (highlighted) is substantially smaller than the other libraries:

How do I get the photographs and the meta data from my older library?
The first thing to check is whether Photos Library.photoslibrary (the 53.31GB one) is Catalina- or Mojave-compatible (Or neither - i.e. it's somehow got corrupted).
If you have access to two Macs - one running Catalina and the other Mojave, try to open the library on both and see which one can open it. Do make sure you're opening the correct library (Since you have more than one):

In Mojave, open Photos &gt; Preferences &gt; Library location &gt; Click on the current library, navigate to where your desired library is located, and select it.

In Catalina, open Photos &gt; Preferences &gt; System library. Navigate to where this library is located, and set it as the system library.


If you don't have two Macs, try this: Right-click on the Photos library &gt; select Show package contents &gt; If there's a folder named Masters, it's Mojave. If there's a folder named Originals, it's Catalina (Be very careful not to make any changes to any of the folders or files, otherwise it'll corrupt the library).
If you open Masters or Originals, the unmodified versions of all your photos will be in there. Latest edited versions (for Catalina) will be in Resources &gt; Derivatives. An easier way to extract the photos along with metadata is with a Mac app named Photos Takeout, but that will only work if your library isn't corrupted.