I found this in the WD My Cloud User’s Manual Page # 116
Hope this answers yoru questions:
"When you attach a USB drive to the WD My Cloud device’s USB 3.0 expansion port, it becomes a network drive. Now the USB drive has the following capabilities:
When you connect an external USB drive such as a WD Passport, a memory stick, or a camera to the WD My Cloud device, you can access it with Windows Explorer or Mac Finder.
The USB drive can serve as a target for backups in WD SmartWare, Windows, or Time Machine.
You can map the drive as a user share drive (Windows) or an alias (Mac).
If a WD external drive has been locked, when it is attached to the WD My Cloud device, it maintains that security and can be unlocked or relocked using the Dashboard.
You can attach a powered USB hub to the external USB port and attach up to seven USB devices to the hub."