Shingled Magnetic Recording (SMR) layers data when written to tracks on the drive, improving data density, but slowing the write process. This would be fine for, say, a DVR that writes once and reads multiple times. For a NAS, it means that a change in the data for a track requires rewriting the track multiple times to layer the data, thus killing throughput.
For WD drives, the telltale part numbers have EFAX in them, whereas non-SMR drives have EFRX in their part numbers. WD SMR drives also have 256MB of cache to support the multiple rewrite of the tracks.