Install this and this only this it the console and driver update:
Allow the Rebuild to happen (if it is Rebuilding) in the OS and then run a verification and repair to check for errors from the console.
The drive may of dropped for any number such as a PSU problem or system lockup but keep a note the of the serial number (don't go by Port) of the drive thats failing.
Thank you Peter, you were right on all counts. Verification running now.