I've been thrilled with my DX4000 so far with it serving as a client backup appliance for 17 workstations in a small business environment.
Unfortunately a few weeks ago I got a notice that one of the drives had failed and needed replacement. The same day I registered the Sentinel on WD's support site and created an RMA with "Bad/Defective Drive" as the return reason. I sent the drive out semi-rush so that the unit would not have to run in degraded RAID5 for very long.
About 3-4 business days after WD had received the drive I still did not have an update on the RMA, not even acknowledging receipt of the drive. I created an online support case requesting a status updated but did not hear back after 2 business days.
Today I called their RMA support line to inquire about the status. I was told that since I registered the DX4000's serial number and created a bad drive RMA under that, they were expecting to receive the entire unit!
Thankfully the support representative was very courteous and knowledgeable (maybe this situation happens a lot?) and filed a new RMA with the drive model and serial numbers. I did note to her that nowhere was it specified in the process that each drive in a Sentinel was considered an individual RMA-able item and that it might be helpful to add this somewhere in the steps.
In Short: If you need to RMA an individual drive from your Sentinel, you must register the drive serial number for the RMA, not the DX4000 itself!