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Hello, my name is Dan.
Usually, resetting the PRAM will do the trick.
Restart then press and hold these 4 keys down, 'Option', 'Command(Apple)', 'R' and 'P' keys, keep them down until you hear 4 startup chimes then let go.
This resets all of the system busses, USB, Optical drive, Firewire, display etc.
As I said, usually this will do the trick. That said, it doesn't always do the trick.
Let me know your results.