Photos for OS X does let you work with multiple libraries, but it doesn't let you import them and combine them all together into a single, unified library. At least not yet. If you have Aperture, however, you can import and consolidate multiple Aperture and iPhoto libraries there first, then get the merged library into Photos for OS X. It's a work around, but it works.
How to import multiple libraries into Aperture
- Launch Aperture on your Mac.
- Click on File > Import > Library...
- Find the library you want to import. (If you don't see it, try looking in your *Pictures** folder.
- Click on Import.
- Repeat steps 2-3 for every additional library you want to import.
How to import your consolidated library into Photos
If you haven't setup Photos for OS X yet, you can simply select the new, combined library as the one to import when you do.
- How to setup and start using Photos for OS X
If you have setup Photos for OS X already, you can simply open the new library like any other. (And once you're sure you're happy with it, delete your old library.)
- How to work with multiple libraries in Photos for OS X
- How to delete your old library after migrating to Photos for OS X
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