It looks like the drive was timing out and causing the controller to not know what to send to the drive indicator.  
On a DELL R730xd server, when a disk drive fails, I see the activity LED(the downside one) blinking green once per second continuously, but this LED status is not described in DELL R730xd user manual(page20, About your system → Hard-drive indicator codes). BTW: the RAID card is PERC H330 set to HBA mode, and I run FreeNAS 9.3 on the server. Pulling out the bad drive and inserting it into another bay show the same symptom, and the drive cannot be recognized by FreeNAS.  Replacing it with a new hard disk works OK. Can someone explain what the very LED blinking pattern means? See the video for the blinking pattern, [URL] .
R730xd drive bay, what does Activity LED slow blinking(green) mean?