You need to replace the /etc/init.d/volatile-media.sh from your HDMU image with the file from the OpenNFR image in flash to get the mounts working properly when HDMU is used as an OMB image. At least that was the reason in the past, would need to test this with an actual image. Or does OpenNFR use some multiboot feature different to the open-multiboot that was originally provided by Gigablue?