It may be a drive letter conflict.
Plug both of them in, then right click on "computer" select manage.
From the list on the left, select disk management. '
From that point, let us know if you can see both hard drives in the list that shows up. If you can, select the one that isn't showing up in "computer" and right click it. From the popup menu, you can assign a new drive letter to it.
Hope that helps, and welcome.