Exporting one Photos library into another to create one combined library

You might consider using the paid version of PowerPhotos to merge the two libraries. Here's a comparison of Photos import vs PowerPhotos merge:

I ran a number of tests with a 3119 photo, 72 video and 97 keyword Photos library with keywords, captions, keywords and locations.  I merged it into an empty library using Photos and PowerPhotos.  This is what I found was imported by each;

PowerPhotos:           Photos:

Albums***                                          -----------

Smart Albums (as regular albums)** -----------

Folders Containing Albums -----------

Captions                                             Captions

Titles                                                   Titles

Keywords* Keywords (some but not all: …

…see Note *)

Original images                                  Original images

Edited images                                     Edited images

---------------                                        Locations

Favorites                                             Favorites

Can detect and exclude                      Duplicates

duplicates upon import or not

Neither method could import/merge projects. 

Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library. 

Notes:

*The Photos app only imported 84 keywords our of 97. PowerPhotos merged 147 keywords. Photos only imported keywords that were embedded in the original file and not those added by Photos. PowerPhotos imported all of them.

**The original library had 90 Smart Albums. All were merged by PowerPhotos but, as indicated above, the Smart albums were brought over as regular albums. No album of any kind were imported by Photos. Also only PowerPhotos imported folders.

The original library had 34 regular albums and 1 folder with some nested albums. PowerPhotos merger all of them successfully.

The original library had 72 videos. Photos imported only 57. PowerPhoto got all 72 in its merge.

dorothy277 wrote: iMac running Mohave. … choosing to export Masters.

I'm not sure what "export Masters" means, but it doesn't sound like something Apple would say to do. Do you have backup copies of these Libraries?

It may be that Photos will eventually generate thumbnails, but my biggest concern is that by opening up the Libraries and dragging portions of it around that you might have caused some Library corruption. Photos depends on the Masters (later called originals, I think) to work with other stuff in that Library. Trying to "merge" Libraries this way will lose a lot of the organization that makes Photos powerful.

Honestly, if I were going to merge two older Libraries, I would use the trusted 3rd party app PowerPhotos ($30) Many of us use PowerPhotos to handle multiple Libraries, and it has very powerful aides to use with Photos.

dorothy277 wrote: … Would it work to just put the albums that compose the archive on a separate drive? I just bought one that is like a very fancy thumbdrive. I will continue to need to download donated photos on the Mac, edit them, and store them somewhere. I will also want to download my own "work" photos. Thanks for thinking about this with me. Dorothy

There's no problem at all in using multiple Libraries if that's what you need. I have a work Library, a Nikon Library, an Old Family Pictures Library, and others. I pick the favorites from each of these and put them in a Favorites Library that's connected to iCloud for synchronizing with my iPhone and iPad. Many of us use multiple libraries, and we use PowerPhotos to make it all work together.

I gather the recent "favorites" from, say, the Nikon Library, and put them in an album that I then transfer to my Favorites album with PowerPhotos. This sounds very like what you are wanting to do. So I know this can work well.

You can add a "Work" keyword to all of your work photos. Also add a "Personal" keyword to all of your personal photos. Do this before you merge the libraries.

After you merge the libraries you can create a coiuple of ssmart album with the following criteria:

Keyword is Work

Keyword is Personal

Each smart album will contain the appropriate photos. As you add new photos add the appropriate keyword to them.

I'm not quite sure what you are meaning. Dragging into a Photos Library can be OK, because it's clear what you want added. Dragging out of a Photos Library is problematic, because Photos doesn't know if you want a thumbnail, the original file, an edited file, a jpg, a TIFF, or what. So it's not good to drag a picture from a Photos Library to anywhere unless you test it first.

If you want to add new pictures that have been files in the Finder, then dragging them in is fine. Photo keeps an exact copy of that file, and you can edit it and organize it with others.

Usually we don't make a new Library to create a project. We would normally make an album for the project within the existing Library, and then we'd drag pictures from the Library view into that album. Pictures can be in several albums without taking up any more space. We can also use Smart Albums to help collect the pictures we want to include.