I'd forgotten all about that question! It's like Apple never considered the possibility that you might connect an iPod and a hard drive with different files and want to keep the union of them.

Similarly, the default setting where Apply tries to copy all of your content to a new iPod when you plug it in, in alphabetical order. I have 300Gb of music, you should know that won't succeed.

Thats just it, they done want to let you keep both files as a way to try and combat piracy (Multiple people syncing all the files theyve bought separately). Its really frustrating, and they will never allow it to happen.