Unable to set Photos system library I moved my Photos library to an external drive due to it's size.  I removed ~/Pictures/Photos Library.  When I launch Photos it asks to choose library.  I select "Other Library" and traverse to external drive where I select the photo library I moved.  Everything works as expected.  I then choose Photos preferences and and select "Use as System Photo Library".  

The next time I launch Photos it cannot find the library and I have to go through the above sequence again.  I removed ~/Library/Prefernces/com.apple.Photos.plist and redid the procedure above.  When I check com.apple.Photos.plist it has the path to the external library.  Yet when I launch Photos it again says it can't find ~/Pictures/Photos Library.

Any recommendations what to try next?
This might be simpler using EasyFind from the App Store [freeware]

A search [including invisibles] for apple.photos immediately after launching Photos holding  Opt ⌥ , then quitting, shows several modified files, including 

~/Library/Containers/com.apple.Photos/Data/Library/Saved Application State/com.apple.Photos.savedState
/private/var/folders/nc/c83zpk8x7kd9qrxrd3nnbfym0000gn/T/com.apple.Photos.librarychooserservice

It might be useful [after making sure you have a good backup] to test out some of those. SavedState is always a prime candidate for this type of thing, I'm less sure about the chooserservice, but it sounds apropos.