I've seen this a couple of times during the life of my DNS-323 post and pre FFP install. What happens is both drives in my RAID config go to sleep after activity, and the central light dims, but the two disk lights stay lit. Waking up the drives and waiting for them to sleep doesn't cure it. Rebooting the NAS does.
Could it be a file lock or something?
V1.08 2x500GB Seagate RAID. B2 hardware rev.