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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:

enter image description here

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:

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How do I get the photographs and the meta data from my older library?

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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 > Preferences > Library location > Click on the current library, navigate to where your desired library is located, and select it.

  • In Catalina, open Photos > Preferences > 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 > select Show package contents > 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 > 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.

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I upgraded to the non-beta edition of Bur Sur, which allowed me to open the newer file (which contained all of my photographs).

Is doesn't answer my original question, however.

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  • The actual answer to your original question is "recover the Catalina-compatible library from your backup".
    – nohillside
    Commented Nov 19, 2020 at 14:40
  • I did not have a Catalina-compatible archive of the file--only the Big Sur one. Original question, which is essentially "is there a way to convert a photos library to an earlier version?", remains unanswered.
    – craig
    Commented Nov 19, 2020 at 15:13
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    This can't be done (and is probably the reason Apple recommends to make a backup before installing the beta).
    – nohillside
    Commented Nov 19, 2020 at 15:15

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