Certain USB drives not being recognized with the OS 3 firmware on the My Cloud is a known issue in this subforum. There are several other threads that discuss the issue with no solution as of yet.
Contact WD Support and start a support case so you can send the logs of your My Cloud to them. The more people that send in their logs will hopefully help WD engineers/programmers find a fix for this particular issue that plagues some of us (me included).