Photos for Mac

Great that it worked for you, Ben.

Photos does not support to merge libraries. We can switch between libraries and work with them in turn.

Merging libraries should be lossless. It should not just be a transfer of image files. We want to be able keep our albums, metadata, and the images with adjustments should be transferred in a way, that will keep the originals paired with the edited versions, so we can revert the adjustments.

There are three ways to merge your libraries, if you prefer a unified library. Which will work best for you, will depend on your system version and your use of iCloud. Which system is running on your Mac? Are you using iCloud Photos?

See: Notes on Merging Photos Libraries, 2019 Version

• If you are using iCloud Photos, you can upload each library in turn to iCloud and iCloud Photos will merge them in iCloud, preserving the albums and the adjustments, while avoiding duplicates. This will take a very long time, but will give the best, lossless merge . Only projects cannot be uploaded to iCloud.
• If you are not using iCloud Photos, but have upgraded to macOS 10.15.1 or later, the full, paid version of PowerPhotos will give a very good merge of the library. PowerPhotos is a very good companion to Photos. I am using it a lot as a second browser window for Photos, for sorting the photos and comparing them, finding and removing duplicates. You can also use PowerPhotos on older versions of Photos, before macOS 10.15.1, but the merging will not be able to keep the originals paired with the edited versions and less metadata can be saved. Upgrade to macOS 10.15.1, if you can, before you start merging libraries.
• Without iCloud or Power Photos you have to do it the hard way. This is Apple's recommendation. (Combine libraries in Photos - Apple Support) Export your photos from your smallest library as the unmodified originals (with XPM sidecar files with the metadata) , then import them to your largest library. Do the same for the edited versions. You will have to recreate all albums and may need to correct the dates again, the locations. I would prefer to keep the libraries separate instead of going down this route.

Before you start to merge your libraries, make backup copies on an external volume. You may need them, if something should go wrong.

The internal folders "database, external, Master.legacy, originals, Plugins, private, resources" are as they should be, but the filename extension of your Photos Library should be "photos library".

Try to rename the folder "photoslibrary" to "Photos Library.photoslibrary", with the correct filename extension . Then it should again be showing the icon with the rainbow flower. once you renamed it, quit Photos, if it is running, then double click the renamed library to open it in Photos.

Hi Ben

The (paid £$30) app powerphotos will copy photos (together with albums and edits) from one library to another. It will also detect duplicates (based on filename and date, or on exact match) as it does it.

The only real alternative is to copy manually via export from one library and import to another. You would have to manage duplicates yourself - or at least be careful about them.

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