I just noticed that Console does give an error message for BitTorrent Sync: "BitTorrent Sync: Failed to notify due to missing registration, queue and reregister" This however seems to be an issue with Growl and not connected to the syncing issue... *New update* Using Kaleidoscope I identified even more incongruencies between the two machines. One has issues with keeping old versions of apps; the other refuses to delete files that have been deleted on the other machine. In other words, BTSync is simply NOT SYNCING dozens of directories in the iTunes folder. Also, the Applications folder was getting a bit out of hand with the indexing (500,000+!!), and to make sure that this giant queue wasn't the source of error, I removed it from my syncing list. This did nothing to improve the problem of the Music directory not syncing.