So, I take it you added a second HDD? Does your BIOS recognize it? Need to confirm that first before we proceed. Verify that the device is enabled in your BIOS and that your power and data cables are securely fastened and then come back with the results.
Does Disk management see the second hard drive? if it does then you need to partition and format it. On Windows 7 Disk Management can be found in control panel, System and security, Administrative tools, Create and format hard disk partitions.
If Disk management does not see the drive then see the previous post by COLGeek