ipod synced with another library

If you were updating automatically previously and the content of the iPod is the same as the content of iTunes press "Erase and Sync" this will link the iPod and iTunes again. If you were updating manually before and your iPod has songs that aren't in iTunes, press "Cancel". Pressing "Erase and Sync" in that case will irretrievably remove all the songs from your iPod. Your iPod should appear in the iTunes source list and from there you can change the update setting back to manual and use your iPod without the risk of accidentally erasing it: Managing content manually on iPod

You can also use a keyboard command to prevent your iPod auto-syncing with iTunes. While connecting the iPod to the computer on Windows with iTunes 7.3 or later installed hold down the Shift + Ctrl keys together. This will stop the iPod from auto-syncing with iTunes and the iPod will appear in the source list. Wait until you are sure the iPod has mounted, and that it will not auto sync and then you can let the keys go. This may take between 20 to 30 seconds depending on your computer. Once you are safely connected you can if you wish set the iPod to manual at that point: iTunes 7.3 Keyboard Shortcuts for Windows

If you have any iTunes Music Store purchases the transfer of purchased content from the iPod to authorised computers was introduced with iTunes 7. You'll find details in this article: Copying iTunes Store purchases from your iPod to a computer

The transfer of content from other sources such as songs imported from CD is designed by default to be one way from iTunes to iPod. However there are a number of third party utilities that you can use to retrieve the music files and playlists from your iPod. You'll find that they have varying degrees of functionality and some will transfer movies, videos, photos, podcasts and games as well. Have a look at the web pages and documentation, this is just a small selection of what's available, they are generally quite straightforward. You can read reviews and comparisons of some of them here:
Wired News - Rescue Your Stranded Tunes
Comparison of iPod managers

TuneJack Windows Only
iPod2PC Windows Only
iGadget Windows Only
iDump Windows Only
SharePod Windows Only
iPodRip Mac & Windows
YamiPod Mac and Windows
Music Rescue Mac & Windows
iPodCopy Mac and Windows
iRepo Mac & Windows (iPhone and iPod Touch compatible)
iPod Access Mac & Windows (iPhone and iPod Touch compatible)

There is also a manual method of accessing the iPod's hard drive and copying songs back to iTunes on Windows or a Mac. The procedure is a bit involved and won't recover playlists but if you're interested it's available at this link: Two-way Street: Moving Music Off the iPod

Go to the YamiPod documents page, instructions for using the software can be found there:
YamiPod Docs

Help me too!
I conected my ipod to someone else's computer and I clicked ok, without reading first that it was a window asking to sync the ipod... I lost all my music files and got instead the ones that were in that pc!!!
Please tell me that I can find all my lost files somewhere in that PC!!!
Please!!!
Thx 4 ur help!
😟

I had this same error, along with about 3 or 4 others. The answer that solved my problem was actually very simple, but it took me a while to figure it all out. Even though iPhoto was not actually syncing with my iPhone, it would launch or make the application active every time I plugged in my iPhone. This is what seemed to prohibit iTunes from being able to manage a successful sync process. All I did was go into the preferences in iPhoto and turn off launching the program when a camera is plugged in. Funny, iPhoto says nothing about the iPhone in this area, but it actually falls in the same category.... think about it, it is a camera and it recognizes all your iPhone images and makes ready for an import. Turning this feature off solved my problem. As I said, I had about 3 or 4 errors, one of which was this one.