My Photos library has about 110,000 photos and library. After upgrading to DP6, launching Photos app triggered "repair" screen. I proceeded with repair and once done, it showed only about 34,000 photos and videos. Panicked, I checked "Masters" folder in Photos library and it shows all 110,000 photos and video. So I duplicated Masters folder to a different disk and attempted to re-import. Well, Photos app prompts that every single one of these photos and videos are already imported. I tried rebuilding the library but nada. I obviously logged it as a bug, but this is pretty scary stuff.