Hi.
You're certainly not the only one to have this issue. Come one Apple, you either have a problem with your product management that don't think that sharing a photo library between users on the same computer is a reasonable expectation or with your developer team that cannot manage file locking and sharing properly. The support of this scenario in iPhoto was already shady so it's clearly not in your agenda to fix this. What's next ? Will you remove support for multiple users on the same computer ? Sorry but my desktop Mac is not like my phone. Is your expectation that everyone in the family should have its own Mac so that we can all happily share our 300GB library using the Cloud library for an outrageous monthly fee ?
And yes I have my library on a separate drive with ownership ignored on the volume. This doesn't work iPhoto: Sharing libraries among multiple users - Apple Support
It has to do with the way you try to do things in the background (photolibraryd) even when photos application is closed without caring with file locking on the DB but I imagine you already know that
8/23/15 10:55:03.788 AM photolibraryd[5427] Failed to open library Photos Library_2.photoslibrary Error: Error Domain=com.apple.lithium Code=54 "locked by another process" UserInfo=0x7fc219c44dd0 {NSURL=file:///Volumes/MAIN/Pictures/Photos%20Library_2.photoslibrary/database/ metaSchema.db.lock, NSLocalizedDescription=locked by another process, NSLocalizedFailureReason=locked by process photolibraryd(4855)}
8/23/15 10:55:03.790 AM Photos[16808] Error: Error Domain=com.apple.reddwarf Code=18 "RDErrorServiceDatabaseOpenError" UserInfo=0x61000007cd80 {NSURL=file:///Volumes/MAIN/Pictures/Photos%20Library_2.photoslibrary/, NSLocalizedDescription=RDErrorServiceDatabaseOpenError, NSUnderlyingError=0x60000025d2b0 "locked by another process"} while trying to open Photos Library_2.photoslibrary
I don't have much hope that this is going to get fixed because I think that you have a different agenda for us for selling more hardware or monthly fees. I'm just hoping someone in the community will find some way to work around it