If/when the drive unmounts with an error, run fsck on it. was it drive sdd1? if so sudo fskc /dev/sdd1
Thanks, I didn't know if having a raid setup affected the way I could check errors in the individual disks, I'll definitely give it a go then.
I had a bit of a look at dmesg, I didn't google all the errors however to me it looks like you possibly have a bad cable or SATA port. You could try swapping the SATA cables and ports (although, don't do it if the RAID isn't rebuilt!) and see if the error follows the drive, cable or SATA port.
There is also some suggestion it might be the mode the hardware is running in or some kind of power management problem.
WD got back to me saying to just send the drive off without any further questioning.. Before I do that I'll check the cables/sata ports as suggested. Would be very annoying if I got a new drive and the problem persisted! I'm away as of tomorrow but will report when I'm back. Cheers