Steve, now this comes from an absolute beginner on this, but the info I got out of the service manual is this:
A USB port is not recognizing devices
1. Completely shut down, then press the power button to start the computer.
2. Use Software Update system preferences to verify that the latest software is installed.
3. For USB, test ports with an Apple keyboard or mouse. If the left port is not recognized check the DC-in flex cable’s condition and connection. If the right port is not recognized check the right USB flex cable’s condition and connection.
4. If the USB flex cables are fine, replace the DC-in board for the left USB port or the backup battery board for the right port.
5. Use Apple System Profiler to verify that the computer is recognizing the bus. If not replace the logic board.
now I am certain that the Apple experts are going to get this answered better for you.