Wai-Wai, my yours does not understand...
To begin with, because this is not the server, and shelves. Therefore, in order to determine what happened, you need to run the control program shelf or go to her web the face and see the presence of error messages.
And by the way, I agree with SyavaSyava
disks actually not exactly the same - same model, but on the second I see features: logo, that is, different disk manufacturers. And most likely, second simply faulty.