The My Cloud is a network attached storage device. It is NOT an external USB hard drive. If you haven’t done so already you should read through the My Cloud User Manual (https://support.wdc.com/product.aspx?ID=904) to learn how to setup the My Cloud and use its various features.
The My Cloud includes a DLNA media server and an iTunes audio server to allow streaming of media to DLNA client devices and to iTunes and iOS devices over the local network.
The USB port on the back of the My Cloud device is intended for the attachment of external USB hard drives to either increase the My Cloud storage space or to provide a location to backup the My Cloud contents. Again the user manual will explain these various options and features and how to use them.