Esa Ruoho has a 11,000-item iPhoto library and an Apple Photos library for OS X that comprises over 6,000 items:
I just spent 6 hours combining three iPhoto Libraries (one on a MacBook Pro, one on an iMac and one on another iMac) via File Sharing between them, and the Fat Cat Software iPhoto Library Manager. So far, so good.
However, since the goal is to push 13,000+ images to iCloud Photos, I’m not sure how to either combine an iPhoto Library and a Photos Library, or how to combine the iPhoto Library imported to a Photos Library, and a Photos Library.
The good news is that this can be done. The bad news is that it takes more effort.
Photos doesn’t allow a simple way to export a library in a format that iPhoto can read. But you can’t merge libraries in Photos yet, directly or through third-party software.
So you have three plans of attack, two of which involve exporting an entire library, which can be problematic, as you will lose albums and other library information that’s not specifically associated with individual media files’ metadata, like keywords or title.
Upgrade your iPhoto library to Photos. Export the current Photos library as files. Select the All Photos album, then Edit > Select All, then File > Export. Pick Export Unmodified Original if you’ve made no changes to photos in iPhoto; Export Photos if you have. Then import those 6,000 photos into the master Photos library, and remove duplicates with Fat Cat’s PowerPhotos ( free for iPhoto Library Manager owners). Make this new superset your master library for iCloud Photo Library.
Export your 11,000-photo iPhoto library through iPhoto’s export options, which are the same as Photos but labeled slightly differently—File > Export and then select Original or a format from the Kind pop-up menu—and import them into the 6,000-photo Photos library. Remove duplicates with PowerPhotos. Then set it as your iCloud Photo Library.
Upgrade your iPhoto library to Photos and make it your master for iCloud Photo Library, but keep the 6,000-photo Photos library separate, and use PowerPhotos to switch between them. Wait until PowerPhotos or another tool allows Photos library merging—Fat Cat’s developer is still working on making the feature happen.
Waiting is hard, but it will provide the best outcome to preserve as much data as possible. Just make sure if you pick option three that you have full and current backups of your 6,000-photo Photos library.
Ask Mac 911
We’re always looking for problems to solve! Email yours to firstname.lastname@example.org including screen captures as appropriate. Mac 911 cannot reply to email with troubleshooting advice nor can we publish answers to every question.