No such thing as a parity disk in raid 5, parity is distributed across the array disks. Odds are it is a hotspare drive, and it is not wriiten to in an array; on my systems I have a backplane which will blip a LED ocassionally to check the hotspare is alive.
Either the 6th is dead, offline or a hotspare or the LED is not working. If you have raid managemnt software within the OS check there; if not restart the machine an go into the raid setup.