Actually I want to use some ARM based development board as usb-device and usb-host at the same time.
So, for usb-host mode I want to read data from inserted usb flash drive into my dev board (scan all files recursively in all directories and check file extension and file headers).
For usb-device mode I want to emulate my dev board as usb flash drive, so that I can insert dev board into my PC via usb port and recognize it like a usual flash drive then open file manager and see some files.
As you could already guess in usb-device mode while emulating usb flash drive it should show all files that passed scanning in usb-host mode from real usb flash drive 
How difficult it is to make?
What dev board would you advise?
Is there any library for this purpose?