Expansion Kit for Sentinel DX4000 4TB in Australia

Hi,

I have a WD Sharespace, under warranty, which has failed.  WD have no replacement units available, so are going to provide a Sentinel DX4000 4TB instead.

I actually want to get 2 x 2TB expansion drives to add to this unit.  The tech specs say that the expansion drive to use is the WDMX004RNN.  However, this appears to be no longer available, either from WD or from any resellers that I can find.

What expansion drive should I purchase to make the unit up to 8TB (Raid5)?

says in stock

http://store.westerndigital.com/store/wdus/en_US/DisplayAccesoryProductDetailsPage/productID.253980500