Don't auto sync because ipod
can only sync with one computer itunes or the ipod
files will be erased.
itunes blocks this function since itunes is the tool with one way to transfer files from itune to ipod. you must utilize other tools to achieve that. There're couple of tools for transferring the files between ipod and computer (for backup)depends on your demands, e.g transfer only songs/videos, transfer photos etc. You can have a look of below review site, hope it useful.
All you need is to get all existing music/videos from your iPod back to your computer and iTunes. Simply use this handy transfer program called Tansee iPod Transfer
Many of my friends are using it for its easy and safe transfer.
With just a few simple clicks, it will help you copy all videos/songs from iPod back to your selected folder on your computer, and then you can easily add those files back to iTunes.
More importantly, this won't overwrite any original files on your iPod. And all song names are preserved.
In this case, all songs/videos (both new and old) are on your iTunes library that you can syn with whenever you wish to without deleting any files.
Here is step-by-step guide on the whole process:
For more iPod support information you can also go to http://www.apple.com