I have 1 of my DNS343 running with 4 x Seagate Barracuda LP ST32000542AS 2TB drives for the past 6 months, running in RAID 5, and have NO issues. fw: 1.04Beta. They work flawlesly.
In another unit I had some WD WD20EADS drives a while back in RAID 5, and they worked fine. But not too long ago, I got a few more, and in RAID, they were useless... Transfer rates dropped 10fold, and they would even timeout occasionally. Thought maybe it was some firmware issue, so I tried the drives in my QNAP TS409Pro, and my Synology DS508 and they were still useless. In Non-Raid, they each seemed to work perfectly, but as soon as they were in some raid config, they crawled. I eventually replaced them with some 1TB Seagate ES.2 drives I had lying around, and the unit is working fine for now.
WD20EARS can be made to work if you manually align the partitions, but I can't be bothered.. Especially if a drive dies, the last thing I need is to have to worry about realigning before rebuilding.. If it is not supported in default firmware, I would not use them.