If you check the WD My Cloud Dashboard does it indicate that a USB is connected? The USB icon looks like this (under the OS 3 firmware) when a drive is connected:
If so, then check the Alarm icon to see if there are any errors reported for the USB drive. The Alarm icon looks like this (under the OS 3 firmware) when there is a notification:
There are some of us who are having problems with certain USB drives not properly mounting with the WD My Cloud running the OS 3 firmware. In my case two different external USB 3.0 drives are not mounting properly with the v04.04.01-112 firmware. Here is one such thread discussing the issues.
https://community.wd.com/t/usb-my-book-not-recognized-reporting-0k-available/96850/46