The thing that is very annoying is that WD manufactured hard drives whch differ from the standard sold ones by the final number of their s/n code. For instance for 4Gb drives
WD4000F9YZ-09N20L1 does not work
WD4000F9YZ-09N20L0 is ok
I guess the differences between the two are simply of firmware recognization to force users to buy hard drives which are almost twice as expansive than WD standard drives. In addition these drives now cannot be found anywhere, for sure for my WD DX4000 they were available in the WD shop since months. I complained with WD shop which answered to ask to the WD support. The WD support told me the WD shop is not owned by WD. The result is that finding replacament or expansion drives for WD DX4000 (as in my case) has been a nightmare