VAR-SOM-AM43 USB OTG as device
From Variscite Wiki
VAR-SOM-AM43 - USB OTG as device
Plug a USB cable to a Linux or a Windows host.
Follow www.linux-usb.org/gadget/file_storage.html and build the backing_file.
On the Target:
$ modprobe g_mass_storage file=./backing_file
On the Host:
Wait for a mass-storage device to be detected.