Moving photos between libraries

Your options are:

a: Use PowerPhotos:

https://www.fatcatsoftware.com/powerphotos/Docs/copying_limitations.html

b: Export to the Finder (File -> Export) and then import to the new library. It is worth noting that v6 of Photos has some bush with exporting, and you have to choose between your Originals and Edited Versions, so you lose the ability to reverse edits.

c: Wait until v7 is released with Monterey OS12, as apparently that version of Photos allows you to copy from one library to another.

Are you using "Optimise Mac Storage" with your iCloud Photos Library? Then none of the suggested options will work well..

• • PowerPhotos cannot move photos from an iCloud Photos Library to another library, if the original image files have not been downloaded from iCloud. You will have to force the download from iCloud by viewing the Photos enlarged in Edit mode, before trying to copy them to another library.
  • Exporting has the bugs mentioned by Yer_Man. And the big draw-back, that the edited versions and the originals will no longer be paired as one photo. You will have to export the edited versions and the originals separately and cannot combine them again in the archive library, you are moving them to.
  • macOS 12 Monterey will certainly making importing from another library easier, but the Monterey preview does not mention what will happen, if the source library is an optimised iCloud Photos Library.
  • Copying the library and pruning the copy as suggested by Tony will work well, but only, if all original items have been downloaded from iCloud. Otherwise the copy will be incomplete.

Are you using the "optimise Storage" feature with your iCloud Photos Library? Then I would turn it off before you split the library, but you would need plenty of free storage to be able to download all originals.

Another option for the first time is to make a copy of your existing library. (This will be a duplicate library but not the system library)

Then from the system version, delete older pictures (say older than one or two years)

And from the duplicate (non system) library delete the newer images (anything newer than the ones you deleted in the system library)

This will buy you time until you can upgrade to Monterey. If your machine is not able to be updated to Monterey, then you will have to use Power Photos for the next time.