When one attaches an external USB drive to a My Cloud, the My Cloud treats the entire USB drive as another Share. You cannot connect one My Cloud to another or even to a computer via USB port.
The Smartware software will not change the backup location to the USB drive attached to the My Cloud unless one manually configures the Smartware software to save to a different location.
Attaching a USB drive to the My Cloud will not affect Smartware or its backups.
Edit to add: If you find Smartware is filling the My Cloud one can change the Smartware settings to reduce the backup version value so the backup (should) take up less space. See the Smartware User Manual (http://www.wdc.com/wdproducts/library/UM/ENG/4779-705057.pdf) Chapter 8, Specifying the Number of Backup Versions, for more information on backup version and how to change the value.