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I get that every now and then, usually if my computer's been off, or in hibernation for a long time. The easiest two ways i've found to fix it, are going into iTunes, and "Finding" where a song is located, then play it. That usually brings it back up. Or going into the Macintosh HD, seeing the drive (even though it's not showing on the desktop, then opening it up. Brings it back.
 
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