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If iTunes is set to automatically update an iPod, it will delete songs from the iPod if they are removed from the iTunes library.
If you want to delete songs off of your computer, but keep them on your iPod, you need to set the iPod to "Manually manage songs and playlists"
With this option enabled, songs will not be deleted off of an iPod if the songs are not on your computer.
For instructions on how to Manually manage songs and playlists, see these links...
Managing songs manually on iPod
Managing your songs manually
This solution works great, especially for people who don't want songs taking up space on their computer. The only problem with this method is that if your iPod is lost, stolen, or needs to be restored or repaired, you will loose your music. The iPod is not designed to be the only location of your music. Please backup your music. And remember, the iTunes Music Store (iTMS) will not let you redownload songs if you lose or delete the song file. For some tips and different approaches to backing up, read through this knowledge base article...
How to back up your media in iTunes 5 or 6
Thank You to King Cole for helping with this tip!
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