Hmm, I couldn't run the diagnostics tool, because WD diagnosis tool saw the array as just one single drive (and then said the cable wasnt plugged in). Not much use there. I'll see what else they've got on their website.
eta: there's a firmware update on WD website for my HDD. It's for RAID arrays, too.
This firmware resolves an issue where a WD1600YS, WD2500YS, WD4000YS, or WD5000YS hard drive is dropped from a RAID set without reporting errors after a period of normal usage. This utility is designed to upgrade the firmware of the following hard drives:
My current version: 20.06C03
Updated version: 20.06C06
*ponders* I'm scared of powering down my PC incase it doesn't start up again. I've got no important things I can't afford to lose on this array, just the OS, and I don't have any live CDs. Guess if it failed, I could dissassemble the array and use the working hdd. So much faff though.