Add another disk

Yes. You can connect any USB external hard drive to the USB port on the back of the WD My Cloud device. You can then use the My Cloud Dashboard administration interface to further adjust access to that external USB drive’s content or backup the My Cloud, via Safepoint, to that external USB hard drive.

Please note that there are currently copying speed issues when copying data/files across the My Cloud’s USB 3.0 port.

http://community.wd.com/t/before-you-pack-up-your-wd-and-return-it-lets-talk-about-copying-speeds/91887