Not sure if your running Unity or Unity Connection. Either way, you should open up a TAC case. They will then determine if the drive is bad and then have it replaced if needed.
Typically, if not all Unity servers have Mirror drives. This means you should be able to pull the drive out and replace it with another drive. IF you are running windows, you should be able to use the HP Array software in the Start Programs applications to check on the array and see what failed.
Run a DIRT or DRS backup soon also.
I had had a few HP servers where the light was nothing if I pulled the drive for a bit, then popped it back in, the RAID was built and it was fine. But it possible, the drive has failed and needs to replaced with the same size/speed part number, etc.
