Download your systme logs, from webGUI go to : status ->hardware -> Logs, then after you download them, try to open the logs with any software that opened compressed files ( ex: WinRAR ) and search the system logs to find a file named tests.log, you may find useful details about sda that lead you to find the drive you want.
This gives me a serial number, which allows me to track the device down to Unit S000, so I'm getting somewhere.
When I come to replace this drive, as it's in a S/W RAID, is it just a matter of pulling and replacing the drive and letting the RAID rebuild, or do I need to do anything else beforehand?