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I have had this problem too ever since I updated my itunes.
Alot of ppl said deleting the SC folder helped them, or authorising and deauthorising their computer, and then restarting their computer.
None of these solutions worked for me.
But this article from yahoo helped me!! Hopefully it will work for you too if the above solutions did not:
The solution that helped me is:
1) Open itunes
2) Connect iphone via cable
3) In "summary" tab under "options", I checked "mannually manage music and videos". Then I clicked "sync". There was an error saying "your computer is not authorised.. etc". That's ok, after that, I unchecked back "manually manage music and videos". Then tadaa! I can now sync my music .