There has been past discussion on using multiple USB drives with a single bay My Cloud. The single bay My Cloud (at least first gen v4.x model) will see multiple attached USB drives and treat each drive as a single individual Share. Some users, myself included, run into problems using more than four drives with one or more drives dropping its connection in the My Cloud. When using multiple drives, make sure to use a powered USB hub even if the external USB hard drives are themselves powered independently.
Here is some proof of multiple drives (mix of flash drives and USB hard drives) being attached to a v4.x single bay My Cloud.
The second image shows the Shares as they appear on the Dashboard Shares page. While it appears to list six, the seventh is barely seen at the bottom below the WD500GB Share entry.
