All USB devices need power from somewhere to work. The 770 will _not_ supply power to the USB port. It's a hardware limitation.

In my experience, every USB device with its own power supply _also_ required power over the USB cable! I think that they use that to detect whether you've plugged them in or not.

In addition, it seems that the 770's USB chipset requires USB power to work as well!

In other words, for the 770 to be either a device _or_ a host, it _must_ get USB power over the cable. Even in host mode, it still needs 5v power.

Virtually every USB device needs power from the USB cable.

I made a magic USB cable that added a power jack in the middle. Hook up 5v to it, and both ends have power. You MUST NOT plug this cable in to anything but a 770! If you plug it into your laptop or desktop's USB port, bad things may happen.

Once you enable USB host mode, and have this magic cable, everything seems to work.

mount /dev/sda /mnt

worked to mount my flash reader.

I did notice that, when it detected USB power, my 770 automatically unmounted the RS-MMC card - even when I was in host mode. This is something that will need to be fixed to make host mode convenient.