The replacement they sent me for the last failed drive makes repeated clicking noises, has noticeable higher noise and vibration levels, and failed the drive self test immediately. A bit more testing and SMART kicked in, saying the drive was unreliable and should be replaced.
They replaced a dud drive with another f@#$ing dud drive! 
The count is now 9 x 300Gb drives, 5 have failed...
I will be asking for a full refund or replacement (of the 4 drives I originally purchased) with a different model.