The WD Sentinel DS6100 has 4 drive bays for 3.5" hard drives. If you put one in and the drive bay light lights up red, you may have put in an incompatible drive. The drive is still usable however and most likely there is nothing wrong with it. Western Digital just wants you do know that they won't support it. They would rather you would use one they have specified.
Anyway, to use the incompatible drive in your WD Sentinel DS6100, all you have to do is go to the Monitor tab of the Windows Server Essentials Dashboard. Look for the drive bay with the incompatible drive and on the left of it, press the Enable button.
You'll get a prompt telling you the drive is incompatible and that data loss may occur. We're not entirely sure how true that statement is. Just simply press Yes to the prompt to start using it. The red light should then go away.
Update: It turns out that you need the latest WD components for this option.