How to Split a Photos Library and Preserve the Albums, Metadata, and Adjustments?

Photo Libraries tend to grow, as we are importing more and more photos and videos, and sooner or later the library will no longer fit onto the volume where ist is residing.

Then we may want to split it into two parts and move the part with older photos to a different volume to free storage. Any external volume where we archive a partial library needs to be compatibel with Photos, prepared like this:

Move your Photos library to save space on your Mac – Apple Support

Let's assume we want to split our library by the capture date - older photos on the external volum, newer photos on the internal volume. For a lossless split, that will preserve the structure of the library, keep the original photos paired with the edited versions, preserves the albums and folders, and metadata, the easiest way would be to duplicate the library by copying it to the external volume. Then delete the newer photos from the copy and the older photos from the original library.

To partition the items in the two libraries into two separate sets, two smart albums will come in handy. Create these two smart albums with "File > New smart Album".

For example, to create a smart album with all photos older than 5 years, and one album with the photos taking during the last five years:

• An album with the new items: Set the smart rule in the smart album to "Date captures is in the last 5 years".

• An album with older items: Set the smart rule in the smart album to "Date captures is not in the last 5 years".

In a similar way you can partition the set of all photos into "Photo is Favourite" or "Photos is not Favourite" or split them thematically using keywords "Keyword is work" or "Keyword is not work":

• After you created the albums, drag the Photos Library to your external volume to create the copy. Name this copy "older than five years.photoslibrary" or any other appropriate name.
• Make an additional copy as a backup, before you start deleting any photos.

• Open the copy "older than five years.photoslibrary" by double clicking it. Open the smart album with the newer, recent photos and select all with ⌘A. Delete them with ⌘⌫ . Empty "Recently Deleted". Your "older than five years.photoslibrary" will now contain only your older photos.
• Now open the original library and delete the older photos from this library in a similar way from the smart album with older photos.

The smart albums are a security line and will help you to split the library exactly in two disjoint parts, and to ensure, that you will not accidentally delete an item from both libraries.

One more thing to consider. If your library is very large, I would delete the photos in smaller batches, not more than thousand photos at once. I had occasionally problems, if the recently Deleted album has been too full.

The procedure described above requires, that the original versions of your photos are stored locally. If you are using iCloud Photos, you have to ensure, that all original photos have been downloaded from iCloud, or the copy of the library will have missing photos, when you open it. If you are using iCloud Photos with "Optimise Mac Storage" enabled, you have to disable "Optimise Storage" before you can split the library or use an alternative method, for example by exporting the original photos from one library and importing them into a new, empty library as described here: Combine libraries in Photos - Apple Support. It will be lossy, however, because you cannot move the the albums or the edited versions paired with the originals.

On macOS 10.15 Catalina you can easily split a Photos Library with the full, paid version of PowerPhotos. But you cannot save the named faces this way: https://www.fatcatsoftware.com/comments/powerphotos_1.7_now_available_with_support_for_macos_10.15_catalina

And even PowerPhotos cannot split an optimised Photos Library without copy errors.