i'm having trouble syncing with itunes after installing the latest update

I have an iPod Nano, 15G capacity, only half full. Since downloading and installing the latest version of iTunes, it won't sync. The error I get is "The iPod cannot be synced. The disk is not available." (I'm also having a lot of trouble with iTunes crashing - not when i'm trying to sync, but rather when I'm not even using it - but I don't know if that's related or not.)

I haven't done anything to try and fix the syncing problem except close and reopen the program and check the settings (which all seem to be OK). I don't know if the fact that all of my music is stored on an external hard drive makes any difference, but I did try moving some of my music to the iTunes directory on my computer's hard drive and it still wouldn't sync, so my guess is that is not the problem.

Help?