RAID is probably Ok: gonne through two verifications without incidents.

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.