One year ago I bought my Synology DS 211+ with two Western Digital 2TB hard disks and I must say I think about disks replacement. The two 2TB WD hard disks in my NAS are not broken or performing poorly but new hard disks are not bad at all. Currently I have the money for two new hard disks for my DiskStation so I do consider buying new hard disks. But hard disks are still expensive because of the flooding in Thailand which is a country that exports a lot of hard disks.
I think I will wait another six months and see if the prices decreasing. I will still buy two new Western Digital hard disks and replace them with my old drives. The Western Digital green hard disks are still the preferred disks because of the large capacity, OK performance and the low power consumption.