I posted this in a different Thread but works here two as I was actually installing my 3rd HDD when I had the problem.
I had the same problem and found a bunch of ways to fix it but the easiest by far is to go
HERE and DL Acronis True Image WD Edition Software. Install and Run it. then scroll down till you see the block labeled Disk Management. In that box click the first choice, Add New Disk. then you will see a list of all drives the one that shows only in BIOS should be highlighted automatically, but if not choose the drive that has no letter assigned.
On the Next Screen you can pick what letter you want for it and a few other things like partition size and stuff. Unless you know what your doing I suggest leaving every thing as it is except maybe the Drive letter. Then that's about it. My new Drive (New 2 g(F: ) ) showed up under My Computer before Acronis was even done running. If you have any other Questions about the program the User Guide PDF is
Here Hope that helps.