Try this (no promises).
Shut the Mac and all drives down, completely. That means powered off.
Now connect the "problem drive" (with power still off).
Power on the problem drive if it has a separate power supply. (If drive is bus-powered, disregard this step)
Now, restart the Mac.
When it reaches the point where the finder loads and displays the desktop, it -may- try to repair the problem drive and then mount it.
Again, no promises that this will work.
Be sure to follow the exact order of events above.