My expertise on this subject is so minimal as to be nonexistant, but I think you might be wise to ignore the "usb device not supported" message. I get this message every time I switch to host mode, using USB Control, whether a device is connected or not. And while some devices don't work, others do, despite the warning.

The N810 does seem to be very picky about which flash drives it will deal with. Some work, some don't, but all are greeted with the "usb device not supported" message.

Please note, this is just my experience, and not necessarily what others have experienced. I am NOT claiming that you don't have a problem, only that the "device not supported" message may be irrelevant to diagnosing and correcting the problem.