When you connect your USB drive to the back of the My Cloud does the USB Icon within the My Cloud Dashboard indicate a USB drive is attached? Does the WD My Cloud Dashboard Alert/Notification icon indicate the USB drive could not be mounted or indicate some other sort of error?
If the USB drive is seen by the My Cloud but does not get mounted (ie no Share appears on the Share page) see the following threads for more discussion.
http://community.wd.com/t/failed-to-enumerate-disk/154447
http://community.wd.com/t/my-mycloud-doesnt-recognize-some-usb-hard-drives/152488
