Degraded

I have an Xserve G5 as a home directory server with two hard drives that I setup last summer with mirroring. This morning I came into a flashing error light on the Xserve. Lo and behold the Server Monitor tells me that one of the drives is running degraded. On the drive module, its disk activity light (the green one) doesn’t blink at all, as if it’s powered on but not being used.

I have never run into this problem before. Has anyone here. Any suggestions? Because it’s an active server for students I won’t be able to touch it until after 2:00…so I am fielding suggestions before I plug away at it.