We got a Dell Precision T550 with two 3TB WD hard drives to be used as a storage server. After I installed Windows 2008 R2 on it, I found that the 3TB drives were recognized as 764GB drives. I found one article on how to fix this problem by installing the latest Intel Rapid Storage Technology driver. This machine has the Intel RAID controller, so I downloaded the latest driver from Intel and it fixed the problem.
Here is the support site for Intel Rapid Storage Technology.
Note that if you click on the Automatically detect and update drivers and software link, it might not detect the right driver to use. Just use the direct link to the driver. At the time of this post, the version is 10.8.0.1003.