Samsung ST2000DL004 (aka HD204UIs)

As your references suggest, rather than relabeling the HD204UI in Seagates new numbering scheme, they took a Barracuda Green drive and relabeled it as a Samsung. As a long time Seagate drive user, I would suggest you try the WD20EFRX. They are fairly new (several months since release) and they are higher priced because demand is currently outstripping supply. Amazon currently has better prices than Newegg (currently out of stock) but I'm not sure about shipping them to where you are.

If you are not familiar with the new WD red label series, this is a line of drives specifically designed for use in desktop NAS units. There were a lot of good reviews when they first came out and they have a 3 year warranty. You might consider getting on or two as spares. If your data volume is increasing, you might also want to consider replacing your 1.5TB with 3TB drives. If you are currently at single redundancy, once you have replaced two of the 1.5TB drives with 3TB drives, you will gain an additional 1.5TB (gross) of data storage.