Have you seen this:
http://community.wd.com/t5/WD-My-Cloud/WD-My-Cloud-hangs-with-external-USB-storage/td-p/868483
There have been other reported problems with USB hanging the MC.
Have you seen this:
http://community.wd.com/t5/WD-My-Cloud/WD-My-Cloud-hangs-with-external-USB-storage/td-p/868483
There have been other reported problems with USB hanging the MC.