I have a similar problem. Previosly I had a seperate router (Netgear R7000) which had a USB connector, where I attached an external hard drive (Western Digital My Book) to be used as network attached storage. This worked fine for years. Now that I have updated/converted to the latest Xfinity 3rd-gen xFi (dispalced the R7000), I can't figure out how to put my network storage back online. The only interface for the external frive is a USB port. I did try a USB to Ethernet adapator and then connected it to the xFi gateway, but the gateway does not recognize it. Open to suggestions..
The USB port on the Comcast supplied gateway devices is disabled. Get an ethernet to USB adapter like this;