As you found out the single bay My Cloud does not officially support copying data from an attached USB hard drive to the My Cloud. As explained above you can accomplish what you seek using a dedicated backup program installed to your computer that would backup from the USB drive to the My Cloud.
Or one could use SSH and the command line interface to issue initiate a copy or rsync command that would copy from the USB drive to the My Cloud. And from there figure out how to create a service that would perform the backup on a schedule within the My Cloud firmware. I’m sure a search of the internet will turn up some general starting points.
A search of this forum using the magnifying glass icon upper right for “rsync schedule” turns up the following thread that should get one started.
https://community.wd.com/t/alternative-to-safepoint-reposted/140685
