1. Your original drive is a 400GB and the new drive is a 500GB. This can sometimes cause problems in building/repairing a RAID.
2. Your original drive is a Hitachi and the replacement is a WD. This can sometimes cause problems in building/repairing a RAID.
3. You replacement drive is a WD 500GB which are known not to play nice in RAID arrays in many servers, including the Snap 4500.
There's a good start for you...