If your library is stored on an external drive, then you need to connect that drive to be able to select and switch to that photo library in Photos app on Mac. If you want to select a different library, then hold down the 'Option' key when you launch 'Photos' app ans select the photo library which you want to open. You can switch between libraries with simple steps. You can have multiple photo libraries in Photos app for Mac OS X. Mac Photos How To Open Two Libraries Simultaneously In One.Mac Photos How To Open Two Libraries Simultaneously Windows.Mac Photos How To Open Two Libraries Simultaneously In Word.