I get this error every time I insert an sd card from my DSLR. It opened my sons external library, and when unplugged, I guess it is trying to connect to it. I know I could create a new library; but I don't want to use photos at all; but I get the message “Photos” can’t be modified or deleted because it’s required by OS X. when trying to delete the app. WTF?
OS X will not let you remove Photos.app. And in their ever expanding quest to make things simpler and easier, certain preferences are hidden.
So open Photos and create a new photo library. there doesn't have to be anything in it, it just has to be there so Photos will open.
Insert the card from your DSLR and photos will see the card and present you with a list of photos on the card.
There should be a checkbox (upper left if I recall correctly) that you can uncheck to prevent Photos from opening when you insert that card.
And the fun continues! You can't turn it off for all cards all at once, you have to do it one at a time for each card you have with your camera.
Perhaps a smart person can find a "defaults write" command to stop this from happening but so far this is the only way I have found to do it.