Save space by moving iPhotos library to external storage device

To check, if your Photos Library on the external volume is working, do the following.

• Quit Photos, if it is running.
• Then double-click the Photos Library on the external volume to open it in Photos.
• When Photos launches, open the "Photos > Preferences > General". At the top of the panel is a line "Library Location", showing the path to your Photos Library. Below is a button"Show in Finder": Click it.

Does this reveal the copy of your library? If yes, click also the button "Use as System Photo Library" to make the new library your System Photo Library, if the button is not yet dimmed.

Check the new library thoroughly, if it is working properly. Check, if the videos are playing and you can see the images enlarged. Check your Time Machine preferences, that the external volume is not on the exclude list for Time Machine, so Time Machine will still backup your Photos Library. Once you have made the first backup of the new library, you can delete the original from your system volume.

what is the library location supposed to show

It will show you the library location of the library you have currently opened in Photos.

You can have multiple libraries, but Photos can only open one of them at a time. If you are nit sure, which library you are currently viewing in Photos, the Library Location line will show you, which library you have opened in Photos.

I suggested this test, so you can check, if you are really seeing the copied library on the external volume in Photos. If you duplicate a library, they will look alike in Photos, and if you want to check, if the copied library on the external volume is working well, you should check, that Photos has really opened the library on the external and is not still showing you the internal library.Photos will fall back on the library in your Pictures folder, if it cannot acmes the library on an external volume.

You can also just rename the library on the computer.