Played around in the RAID controller and found an option to add the new disk to a config called "Raid Ready" so I've done that. Maybe the drive needs to be in that for the array to grab the new drive and rebuild? Any thoughts?
Thoughts: see if the procedure is documented in the User Manual. The RAID section is usually in the back of the manual. You can refer to the PDF manual if you don't have the paper copy handy. Otherwise, I'd recommend referring to Asus Technical Support. -- Making life hard for others since 1977. I speak for myself and not my employer/affiliates of my employer.