Arrgh - am a fool (of a took)!
hadn't realised the disk names under "general storage" on the web server were hyperlinks-showing (amongst other data) the SMART status.

Have found the failing drive and have disconnected until I can get a replacement in.
Thanks for the reply, mikeiver1-usage wasn't an issue in this case.
Would still like to know how to turn the alarm off tho: it was driving the household nuts while diagnosis went on.
Any ideas why disabling SMART on the drive didn't stop the alarm-and what could have?
Many thanks.