You will need the Dfu-util to make a backup of your existing image.
As when flashing, you will need to be in U-Boot in the NOR Flash.
./dfu-util -a kernel -B -U good-kernel.bin ./dfu-util -a rootfs -R -U good-rootfs.jffs2
Here is what my successful output looked like...
on the host PC:
$ sudo ./dfu-util -a kernel -R -U good-kernel.img dfu-util - (C) 2007 by OpenMoko Inc. This program is Free Software and has ABSOLUTELY NO WARRANTY Opening USB Device 0x0000:0x0000... Claiming USB DFU Runtime Interface... Determining device status: state = appIDLE, status = 0 Device really in Runtime Mode, send DFU detach request... Resetting USB... Opening USB Device... Found Runtime: [0x1d50:0x5119] devnum=12, cfg=0, intf=0, alt=3, name="kernel" Claiming USB DFU Interface... Setting Alternate Setting ... Determining device status: state = dfuIDLE, status = 0 dfuIDLE, continuing Transfer Size = 0x1000 Resetting USB to switch back to runtime mode
on the FreeRunner:
DFU: Switching to DFU Mode DEVICE_CONFIGURED: 1 Starting DFU Upload of partition 'kernel' DEVICE_CONFIGURED: 1