How to put Photos.app main library on an external disk?
You can change the library location from the Preferences window on the General tab. Photos &gt; Preferences... and select the General tab.

At the top where it says Library Location: click the Show In Finder button:



In the Finder window that opens, move the Photos Library.photoslibrary to the place where you'd rather it exist. This can take some time if it's a large library.

Once it's moved, double click on it in Finder and your Photos app will update and point to the new location.