Why won't my iTunes let me sync my songs?

I have restarted my laptop, un-installed iTunes then reinstalled it again several times. And after I do any of them I will plug my iPhone 4S into the USB (with cable) port and it'll start syncing my music but then a notification will pop up and say my music can't be found. But all my music is still in the iTunes. I don't get it?

That usually happens due to some asserts that prevents the pc from reading the device or transfer files from it. Try this from another pc or reinstall the drivers.

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