I have a use case where I want a virtual hard disk (particularly, on a freeNAS installation) to act as a USB mass storage peripheral for an external device, such as a phone or video game console. My idea is to do away with all the USB storage devices in my setup and move to virtualizing them as part of my NAS and have them be a painless part of my regular backup routine.
I realize that USB is not a symmetric setup, so I'd need a slave port on my NAS. I also realize this is something done on the actual USB controller card, so there's no real way to just "use" a USB port as a slave port.
My question is, is there any sort of method of emulation or conversion, through hardware, software, or some combination thereof, to achieve this functionality? It seems like I can use the linux usb-gadget api in mass storage mode if I can get a slave port to work. However, I can't seem to find any sort of expansion card with USB slave or USB-OTG support, and I know of no other hardware solutions that I could use in the middle of this setup.
Does anyone have any pointers?
Edit: Would a device like this work?