Dan Wieder wanted to track down a mismatch for his daughter between the images stored on her Mac and what was being synchronized via iTunes to an iPad:
Looking in the album, she can see it has 1,308 photos. But when it syncs to the iPad, only 1,026 photos get synced. I tried the usual things with removing the photos from the iPad and resyncing, rebuilding the Photos Library, etc. So far, it still said 1,026 photos were synced.
I asked Dan to check whether or not the images were externally referenced, something that Photos allows, but which isn’t always obvious. And which shouldn’t have made a difference in syncing.
Read more here:: What’s hidden in plain sight? Hidden media in Photos for OS X