Disaster Strikes! Advice requested!

If a disk fails completely, the led will be blank. I have had this on two different disks. Normally, a disk will give you some indication of degradation of the platter and you will get warnings. However, in the case of a complete mechanical failure or a failure in the on board electronics, it will just die on the spot. You will see a similar reaction if you have a functional healthy array and are increasing the size of disks. When you hot remove the first one, the light will go out and it will be reported as dead.

When I lost my first one like that, it was overnight on a two drive array. I came in to the computer room to only one disk light. I shut the unit down normally and removed the first disk. It was cool to the touch. I then removed the other drive and it was warm to the touch. This confirmed to me that the first drive was indeed dead and I replaced it and rebooted the NAS. I was concerned because the drive that failed was only a month old. After the resync, they ran 24/365 for three more years before being replaced by larger drives.