My Book Essential Drive - 2TB - Can't get it to work AT ALL!

I’m having difficulty getting two WD 2TB MyBook Essential drives to work together on the same computer (Windows 7 loaded).  when I unplug either, the other works.  When they are both plugged in, only the first one plugged in works.  Disk manager says “This disk is offline because there is a signature collision with another disk that is online”.  Any help would be appreciated…