iPad attempting to copy to disk Macintosh HD failed unknown error -4

This has been going on for months and months. Started around the time of IOS7, but has persisted through all updates since then. Basically I can't get through a clean Sync of my iPad3 (Wi Fi only 64Gb). It seems to copy files to the iPad ok, but possibly not to the Mac. Once the Sync fails it sits there forever trying to finish. I always have to cancel and eject.

I have tried rebooting Mac and iPad, tried repairing permissions on the Mac hard drive, tried praying to Jobs, nothing seems to work.