Hi. I have installed two 2TB 5900rpm Seagate Barracuda ST2000DL003-9VT1. I have set 500Gb in RAID1 and 3Tb in JBOD. They have been running for one month so far, without noticeable errors or incidents.
Note: even though the DNS-320 lists my drives as Model No. ST2000DL003-9VT1, I bought them at BestBuy as ST320005N4A.