I'm currently experiencing the same issue with my iPod touch (5th Gen iOS: 8.1.3 , itunes version: 12.1.0.50).
About 9 hours ago, I tried to simply add some music only to find that the "preparing to update" message appeared. While trying to correct the issue, I noticed that itunes somehow managed to delete about 20GB's worth of music. The music that was deleted remained present under "On my device > Music", but when I selected any of the deleted songs, an exclamation mark would appear next to them.At this point, I tried adding more music and my ipod would never update. I then restored my iPod and began the process of adding music again. Halfway through re adding music, iTunes decided to delete the music once again. I'm trying to add music for the third time, and im now back at the same "preparing to update" issue. This is really discouraging and its making me reconsider purchasing an iPhone 6 (Or anything for that matter). Note: I have a 2nd Gen iPod touch that works flawlessly when it comes to adding music however, I'm now hesitant in connecting it with iTunes. Not a good look...