Think this "RAID 5 + spare" that you are referring to is RAID 6... For 2x2TB to 4x2TB RAID 1 to RAID 5 conversion, I don't think it should take that long. Perhaps your NAS has a relatively slow processor. Too bad you are already in the middle of the conversion, otherwise, I would have suggested that you move the data into a different box and build the RAID 5 by wiping out all the disks.