Having had this issue plague me on just getting photos onto my wife's iPhone 5S to get 500 photos to her Apple Watch (eventually gave up and just shared more photos through iCloud with her). I subsequently attempted Robster50's solution and it did actually work (from my iMac) on the first attempt! Just proves something is screwy with the software!
Since then I have subsequently entered the nine circles of **** trying to do an encrypted backup of my iPhone to restore to a new iPhone. I've had the 'iPhone Disconnected' issue, the iTunes Crash issue, the iPhone sync failed to finish error, iTunes restore about half the apps and no content (giving it over 8 hours) .... iTunes just cannot cope!
Eventually I removed everything but the Apps (all podcasts, photos, music, TV Progs, Movies, etc) and did full sync on the old one (due to failures/crashes took about 3 fresh reinstalls of iTunes from DMG download) and then an eventual restore of just the Apps and a reinstall/re-Sync afterwards for other content (again - took about 2 more iTunes reinstalls from scratch).
Something is seriously wrong with this process/software but the only way to carry across App/Fitness /Activity/Watch data etc. I have this pain again to look forward to when I pass on the old phone to replace my wife's current phone. At least I have found a way to bludgeon my way through...