I had a little success with my boxer as usb drive... I guess you should do somthing like:
# modprobe g_file_storage file=/dev/hda1
this mounts my CF card, I couldn't get my SD (/dev/mmcblk0p1) to work this way
to stop the module write:
# modprobe g_file_storage -r
though I must say that using usb networking combined with samba worked much better for me.
YoG.