

(If it’s dimmed out, that library is already set as the System Photo Library.) Wait for photos to upload. In Photos Preferences General, click Use as System Photo Library. Now, starting with the smallest Photos library and working up in size, follow these steps for each library you want to merge: Double-click the Photos library to open it. This becomes your default library, or the System Photo Library in Apple's parlance. When you first set up Photos for Mac, it asks you to choose a library. If you have divided a large photo library into small, more manageable libraries, Photos for Mac makes it easy to work multiple libraries. Then the merged library will download to the library you have currently open as your iCloud Photo Library. The only automatic way to merge libraries in Photos is to upload both libraries to iCloud, one after the other, and they will merge in iCloud Photo Library.

