Cannot access Photos Library, cannot repair Photos Library, cannot find Masters in Photos Library Contents

Have a suggestion. I have never tried doing this with a Catalina-to-Mojave transfer and have no easy way to test, so I can’t guarantee it will work. I know it works with Catalina-to-Catalina transfer. MAKE A BACKUP OF THE CURRENT LIBRARY FIRST.

1. Launch Photos app with the Option key pressed down;
2. On the dialogue that appears, click on Create New;
3. On the next screen add a suffix to the new Photos Library name, so you can easily tell it apart from other libraries;
4. Drag the “Originals” folder from the current library package onto the empty screen of the newly-created library.

Images should start appearing in the Photos window. If there are issues with any individual files, you’ll get a “Cannot Import x Items” list.

Easier than manually dragging and testing one item at a time...

Originals = Masters.

When you opened your photos library in Catalina, the library was converted to Photos v. 5.0 format. That’s why you can’t now open it with the Photos version that is installed with Mojave.

Make a backup of the current library, then retrieve the most-recent library backup that was made under Mojave. You’ll still need to find and transfer whatever new images were added since that backup was made.

Do you have iCloud Photos Library enabled? It would make going back much easier...

How about iCloud Photos Library being enabled?

Other than a manual transfer, it might be worth your time to check out the PowerPhotos app. It’s designed to manage multiple libraries and to merge libraries. Usually, it’s merging an older library with a newer one, but it might work in reverse, too. Please contact the developer to confirm - it’s a $30 app.

https://www.fatcatsoftware.com/powerphotos/