Running out of space on your Mac. You can help free up space by moving your Photos library to an external drive. Here’s how:
Connect an external drive to your Mac.
Move your existing Photos library from your uUser folder’s Pictures section to an external drive of your choice.
Select your desired Photos library from the external drive by option-clicking Photos as you launch it.
Note that if you’re syncing to iCloud Photo Library, you must have the external drive plugged in to sync or view your library. You can’t create a second Photos Library that has iCloud Photo Library enabled on your Mac.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)