This tip was actually discussed in the Mac Answers section of Basics4Mac a little while back. The reader had an iPod that was not being recognized by iTunes. You can read the entire Mac Answers topic here.
Occasionally an iPod will misbehave, either in its own workings or in how it works with iTunes. Apple has a help topic on its support site called The iPod 5 R's. It walks you through 5 steps (which all start with the letter R) that will fix most problems that afflict iPods and save you a trip to the Genius Bar.
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