It may be worth reading this:
http://community.wd.com/t5/WD-My-Cloud/USB-drive-transfer-speeds/m-p/861040#M31056 and this:
http://community.wd.com/t5/WD-My-Cloud/Slow-Transfer-Speed-over-USB/td-p/651477
And the User Manual, poor as it is.
Interesting that Larry suggests the WD apps invoke a local MC to USB transfer; I found a Linux transfer using SSH root login and cp to give about 3MB/s.