Yes - POST means POWER ON SELF TEST so after the bios screen and before the NAS-R3 loads.

This issue could be related to some hardware issue memory, disks, RAID, power hit, bad cache.... So what can happen is that the RAID may state the values are ok but our Logical Volume manager may see something different in the sector size or other that the controller is not reporting. I would be on this like hot cakes! I have seen drives go offline then places the controller in degraded mode then you better hope that you have a good hot spare (even that can bite u) or RAID 6.

Anyway I would download the logs from the controller and check them w/ there engineers to be on the safe side.