As S-n-M suggested...you are probably having harddrive failures. Spinrite is a great program....use it to recover the lost drive's stored info/data and you should be back up with the new drive.....

The power failure or HARD SHUTDOWN could have caused this so I would be cautious as to the use of the other drives in the array....you should not be using the external drive as part of the array but as external storage only.