Photo Library
I uploaded an old iPhoto library to my computer, and all the other photos that were in Photos were deleted, but still remain in iCloud. How do I get both libraries to show up in Photos. Thanks!
MacBook Pro with Touch Bar
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Hi
Do NOT try to import one library into the other. That will result in a whole mess of problems (including importing thumbnails and previews as separate images)
If you want to merge two libraries:
This is the officially documented way. If you choose this method you would probably want to merge the photos from the smaller library into your larger one.
This article describes how.
https://support.apple.com/en-us/HT209528
The problem with that is you can either copy edited versions or original versions - but if you copy both, then they will appear as duplicates in the library.
Alternatively, you can get the paid app (power photos) which will copy photos from one library to another, complete with metadata, edits, and albums.
Finally - you can use iCloud. You'd have to pay for a month of enough space for the combined merged library, but this will be cheaper than power photos, unless you want to have the facilities that power photos offers into the future.
To use iCloud you sync the first library with iCloud, then set the second to be the system library, and that will sync too, merging it's contents with the first in iCloud. Once you've done that you'll have both sets of photos in the second library, and you can stop using iCloud if you wish.
This user tip describes how to merge in more detail, including how to use iCloud. https://discussions.apple.com/docs/DOC-250000662