If md gets a read error when the array is not degraded, it generates the data from elsewhere and tries to write it out. If the write fails, then the drive is marked faulty.
It has not always been that way, but it has for a couple of years.
Now that I think about, there is probably still room for improvement. If it is kicked due to a write error, and it was a cable error, it would be nice if we could re-add the device and it would recover based on the bit map. I'll add that to my list....
(sorry, I didn't read the first part of your comment properly before - I only read the second half and was responding to that. I should learn to rad better ;-)
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