Please also check this post which was the way I overcame the problem of the silly compatibility restriction (even more silly now that that the compatible hard disks cannot almost be found anywhere even if you are willing to pay the extra money)
On one of my two DX4000 I am using this solution and it works nicely, however I would not advise it if you are running the DX in an office environment. In a home environment with the fan always at full speed it could be a little bit too noisy. However the RAID control is much better than the poor one of the original DX4000 (which sometimes does not work as it was my case and at least in another case of a user who recently posted on this forum trying to expand the DX4000)
