Move Photos App and data to external drive to make room on main drive
All you have to do is move it.


Plug in your external drive
Copy (click and drag) your Photos library from your internal drive to your external drive
When complete, hold down the Option key and launch Photos.
In Photos, select Photos > Preferences, and in the General tab, click Use as System Photo Library.
Once you have confirmed that the library is there and is working, delete your old library off your internal drive