I suspect that your laptop does not actually have 2 hard drives but instead has a single hard drive that is split up into 2 partitions that Windows sees as 2 drives. The 2nd drive is more than likely a restore image and the laptop can not boot from it.
If you plan to replace your hard drive, you will want to use software to "clone" the existing drive to the new drive so you don't lose any data or your installed programs. Here is a free one that I used recently when I upgraded a laptop from a 250GB drive to a 750GB drive:
HD Clone- Free Edition