One of the bays on my 2-Bay Mobius displays a red LED and will not mount the drive.

If two drives are inserted and one mounts without issue, swap them around and observe the behavior. If the behavior still persists on the same bay, it is a sign of a possible faulty Mobius unit. If the problem persists in the other bay, it is sign of a possibly faulty hard drive.

Contact the support team directly if you suspect the problem lies with the Mobius unit.