Sep 10th, 2009 (B)

The disk was indeed faulty, but you could only tell after asking the WD diagnostic tool to fill the whole thing with zeros (all other WD-provided tests passed OK).

The take-home message from this RAID-disk story is that the controller is dependable: it correctly identified and threw-out of the array a disk that could have passed all the standard vendor-provided tests. So: if the controller says “throw it away”, throw it away. Don't think, don't hesitate, just replace the darn thing and get on with your calculations.