Externally Powered USB OTG - Nexus 4. "OTG" in this case means "On The Go", it's a method of being able to plug something like an external hard drive or USB thumbdrive into the phone's USB port in order to have some additional storage. The Nexus 4 doesn't supply power on the USB port so a Y cable is needed that can be plugged into a powered USB port. It's cumbersome but if you have an external battery pack it would work fine for moving files on and off the phone.