the same problem here (on xpenology 5592u2 and on original DS414 (!), both using latest dsm), after changing from RAID5 to RAID10 seems to be OK, well i know this is not a solution...
btw http://forum.synology.com/enu/viewtopic ... 4&t=103332 or http://forum.synology.com/enu/viewtopic ... 19&t=83280