Connect your iPod to your computer. When you get the message that it is linked to a different library and asking if you want to link to this one and replace all your songs etc, press
"Cancel". Pressing "Erase and Sync" will irretrievably remove all the songs from your iPod. When your iPod appears in the iTunes source list change the update setting to manual, that will let you use our iPod without the risk of accidentally erasing it. Check the "manually manage music and videos" box in Summary then press the Apply button. Also when using most of the utilities listed below your iPod needs to be enabled for disc use, changing to manual update will do this by default:
Managing content manually on iPod
Once you are safely connected there are a few things you can do to restore your iTunes from the iPod. The transfer of content 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 (iPhone and iPod Touch compatible)
iPod2PC Windows Only
iDump Windows Only
SharePod Windows Only
iPodRip Mac & Windows
YamiPod Mac and Windows
Music Rescue Mac & Windows
iPodCopy Mac and Windows
Floola Mac & Windows
iGadget Mac & Windows (iPhone and iPod Touch compatible)
iRepo Mac & Windows (iPhone and iPod Touch compatible)
iPod Access Mac & Windows (iPhone and iPod Touch compatible)
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 or iPhone to a computer
There's also a manual method of copying songs from your iPod to a Mac or PC. The procedure is a bit involved and won't recover playlists but if you're interested it's available on page 2 at this link:
Copying Content from your iPod to your Computer - The Definitive Guide
Whichever of these retrieval methods you choose, keep your iPod in manual mode until you have reloaded your iTunes and you are happy with your playlists etc then it will be safe to return it auto-sync. I would also advise that you get yourself an external hard drive and back your stuff up, relying on an iPod as your sole backup is not a good idea and external drives are comparatively inexpensive these days, you can get loads of storage for a reasonable outlay.
