I know it's an old question, but, as it might be an answer for someone, I wanted to say I'm giving FreeFileSync a try, an opensource application bundled with another utility, named RealTimeSync. This last one triggers a command (presumably, FreeFileSync, but I believe you can actually execute any command line) whenever the contents of a folder is changed (or when that folder becomes available).
I will update outcomes, but, at first glance, looks promising (despite the scarce and windows-oriented documentation).
Update: seems to work well. I set RealTimeSync to watch for availability and/or changes of two folders (source and destination) and run a Sync job when that happened, and all went well. Deletions propagated, too, with a (I think) nice feature: deleted files (those deleted by FreeFileSync as a result of the syncing) go to a folder inside trash (or inside a folder of your choice), named with FFS+time-date stamp.
Still have to test it under hard conditions (large syncing jobs, over a large amount of files) to check speed of file comparison. Will do tomorrow.