Photos Can't Find System Photo Library

I recently updated my Macbook Pro to El Capitan 10.11.6 (newest my Macbook can use). My photo library has always been on an external hard drive and before the update, had no trouble using that library when I opened Photos. Now, everytime I restart my computer I get an error that "Photos cannot find Your System Photos Library". Once I click "choose another" I'm able to select the library on my external hard drive and then in preferences it is already set at System Photo Library. That works fine until i restart my computer and then get the same message and have to re-select "choose other" and the library. I've tried repairing the library and still get the same message. I had no trouble with this prior to the update.