Question: Photos cannot find the System Photo Library
I'm using Photos version 3.0 (3251.12.190) on a late 2013 Mac Pro running Mac OS 10.13.3. I have an external Drobo drive array connected via Thunderbolt where my Photos library lives. I've noticed pretty much since I upgraded to High Sierra that when I first launch Photos after rebooting, I get an error message saying that "Photos cannot find the System Photo Library". When I click "Open Other..." however, I get a list showing me the System Photo Library, and if I select it to open, Photos starts normally. Every time I open Photos after that, I will get the "What's new in Photos" splash window. That is, until I reboot, when it can't find the library again. This has coincided with iTunes syncing all of my photos to my iPhone whenever I add any new images to Photos. This happens even after I have relocated the library for Photos.
None of this behavior is fatal, but it is certainly annoying (especially the waiting for 5000 photos to sync to my iPhone every time I add 1 new one). Any fixes or troubleshooting would be much appreciated.
Mac Pro, macOS High Sierra (10.13.3)