Like you guys, I use an external USB drive to copy my library from the primary machine to the secondary. Whenever I'm doing show prep, I always use the primary to download any tracks (usually from amazon) needed. After I've finished downloads and have reindexed the player database, I stick an external on and use robocopy (I have an icon on my desktop that fires off the script) to differential copy anything new from the primary. I then plug that drive into the 2nd unit and use robocopy to pull updated tracks from the external to the 2nd unit. I then reindex on that machine, which brings both machine in sync. Just before the gig, I put all of the special event songs at the top of the playlist on both machines. On the 2nd unit I then add one dance song and one dinner song. During the special events I'll fire off #1, then fire off the same song on #2 (and sync). During dinner or dance sets I'll loop the dance/dinner song on the 2nd machine. Kinda overkill but if/when something dies on machine #1, back in business is only as far as turning up the volume on channel #2.