And of course, space isn't unlimited, so I don't want to keep a huge Aperture library and an i Photo library too.

So far, my use of Aperture hasn't made anything better or easier, it's just complicated an already complex situation, but I'm sure that's because I've done something stupid.

To avoid loss of important data, you should backup the libraries before the process of merging.

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Unfortunately, the apps can't be used interchangeably.

If you make edits to a photo in i Photo or Aperture, those changes will not sync to Photos.

All other i Photo libraries are present and still can be accessed through your Mac.

After you confirm that your unified image library works correctly, you can remove all other i Photo libraries out of Mac’s hard disk.

First select the largest library and wait for it to completely migrate from i Photo or Aperture (this could take a very long time, depending on how many pictures you have in your largest library).

Then, you can begin the process of migrating other photo libraries.

This only works with i Photo 9.3 or later and Aperture 3.3 or later.

Once merged, you can then open Photos and migrate the single, unified photo library.

If you have already opened and migrated libraries into Photos, you can simply delete the old library once you've confirmed that the new, merged folder was properly uploaded.

If you don't own Aperture and want to combine multiple i Photo libraries, it's a bit trickier.

Similarly, edits made in Photos for OS X will not sync to i Photo or Aperture.