Computer not recognizing 2nd of the same WD mass storage device

I have two WD 3tb external drives. They both work but only one at a time. I am running Windows 7. I have renamed both drives but still no go. I have previously have had a 2tb and one the 3tb drives running at the same time with no problem. I had to upgrade to the 3tb from the 2tb because of the size of my projects I am working on. Does anyone have a solution?
 
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