[meaubry]

Try taking the motor out of the drive system and testing the motor with no load on it.
Using a power supply, apply a small voltage (9V) and determine if the motor shaft rotates. Flip polarity between motor leads and re-test. This will determine if the motor itself is the problem.
Do not take the CIM apart - it isn't necessary to determine if it is functional or not.
I expect the CIM is probably good, because it runs in one direction, but not the other.
That would point to a speed control, or potentially a programming issue.
You should also try swapping drivers controllers too, just in case the joystick or whatever, isnt working properly.
If you don't want to or cannot get the CIM out by itself, then get the robot off the floor and determine if there is a mechanical back drive issue by rotating the wheels by hand.
If the wheels do not move, check the rest of the drive train for anything that might be interfering with the whatever is driving the system (chain, belt, etc).
The CIM motor output shaft should rotate freely in both clockwise and counter-clockwise directions. It isn't like the window lift motor, that doesn't allow it to backdrive freely.
Hope this helps,
Mike