Since many modern Macs come with limited internal storage, it can be handy to move space-consuming items like your OS X Photos library to an external drive. Thankfully, it’s easy to do this. Quit the app, attach an external drive to your Mac, and then:
Make a copy of the Photos Library by dragging it from the startup volume on your Mac’s hard drive to the external drive.
When the library is copied, hold down the Option key and launch Photos.
When you see the above message pop up, click on delete incomplete items.
In the Photos app, go to Preferences and under General, click Use as System Photo Library.
Then go to System Preferences > iCloud > Photos > Options, Select iCloud Photo Library to turn that back on.
Test the library and when you're sure all is well, trash the one on your internal HD to free up space.
Voila. That’s all there is to it.