Thanks for the input.
Sorry if I'm not understanding... I've been able to copy my IPHOTO library to my external drive, no problem. Once I update to 10.10.3, updating iPhoto to Photos, the PHOTOS library will not finishing to my external HD.
You're saying I can't copy the library to an external drive unless its HFS+, but I've been doing it for years with my iPhoto library by simply dragging and dropping the library from my computer to the external HD (which is not HFS+).
What makes the PHOTOS library different that it will not complete the transfer? It gets to around 50gb out of 55gb and gives me the above referenced error (unexpected error has occurred).
So your statement that HFS+ is required doesn't make sense to me when I've been doing it with iPhotos for years... Thanks.