Thecus N4100EVO RAID 5 fails to rebuild

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RAID 5 composed of 3 1TB WD RE4 HDDs degraded during warranty claim process (guys at warranty claim department accidentally removed one of HDDs during testing procedure - soreee). Now I'm trying to rebuild my RAID and my data - unfortunately with no success. By the way my N4100EVO has been sent back to Thecus 30 days ago and returned to me with comment, that no faulty on the device has been detected. If this is not faulty, then what else could it possibly be ? !!!