My Book Thunderbolt Duo 4TB no longer recognized by Mac

Since two days my external thunderbolt drive is no longer recognized by the Mac. Scanning the drive with WD Drive Utilities hangs at 85%. Mac OS X Disk Utility sees one of the two drives, the second is designated as No Name and cannot be mounted.
Drive Utilities says the My Book is configured as Raid 0 (striped), but that is incorrect. It was configured as Raid 1 (mirrored), and I am certain of that because I had only 2TB of the 4TB capacity. Using an iMac 27" with Mac OS X 10.11 El Capitan.
Any help/thoughts?
Thanks folks.


Just as a troubleshooting step make sure the drive is plugged in directly into a wall outlet to make sure it is not a power issue.

Hi @Hamlet, thanks for the response, but this doesn’t help unfortunately. The power is already plugged into the wall.