I do the same. Normally have a 5TB Seagate Backup Plus Hub attached to the USB port of the My Cloud to use as a Safepoint backup for the My Cloud and additional My Cloud storage. One can enable USB Content Availability under the My Cloud Dashboard > Settings > General > Cloud Settings to have the additional USB storage space reflected in the Capacity section of the Dashboard Home page.
There are some issues however with using the USB port. One is speed. Lots of complaints (do a forum search) about poor USB 3.0 speed when copying to or from a USB 3.00 hard drive attached to the My Cloud’s USB port. Then there is the issue of using multiple USB drives. While the My Cloud User Manual indicates one can use up to seven USB hard drives when one uses a powered USB hub, some of us have found that the My Cloud has trouble properly mounting and accessing more than four or five USB hard drives at one time. See the following discussion for more on using USB hubs:
http://community.wd.com/t/my-cloud-and-usb-hub-powered/168885
