How to put Photos.app main library on an external disk?

Solution 1:

You can change the library location from the Preferences window on the General tab. Photos > 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.

Solution 2:

You do it exactly the same way as iPhoto:

Make sure the drive is formatted Mac OS Extended (Journaled)  

1. Quit Photos  
2. Copy the Photos Library from your Pictures Folder to the External Disk.  
3. Hold down the option (or alt) key while launching Photos. From the resulting menu select 'Choose Library' and navigate to the new location. From that point on this will be the default location of your library.  
4. Test the library and when you're sure all is well, trash the one on your internal HD to free up space.  

The option in Preferences allowing designation of a System Library refers exclusively to a scenario where a: there is more than one Library and b: one is using iCloud. As only one library can be used with iCloud, that's how you nominate your preferred one. It has no role in moving the Library.

Regards  

Solution 3:

The top answers are good, but the completely correct answer is to manually switch between the libraries when necessary by repointing the app to the library you wish to manipulate. This implies you need to manually import your photos from the laptop library when you want to archive them to the external library (photos older than 1 yr per your example). I used apps that were developed for the iPhoto app in the past to do just that. They automated switching libraries and I archived via importing into the "master" library. Unless metadata can be lost through an import or some type of issue similar to that which I am not aware of, I don't see anything wrong with this tactic.