Seems like the limit is add about 635 songs (?) I have 670 songs in my lirbrary and the synching gets stuck at 635 from 670 songs. I tried repairing and resetting my watch. I have the Series 4 version and I still have about 6gbs left. There also some songs that just wont play any audio on my watch (when no internet is avaiable) edit: I shrinked my library to 600 songs and all of them seem to get synced to the watch but there are still songs that wont play on the watch when there is no internet connection. Anyone know whats going on here?