It's itty itty bitty. About 64MB, and some of that U-boot'll be using.
If you have a shell, you should be able to do something like the following:
dd if=fs.img of=/dev/mtdblock6 bs=1024k
Then wait awhile. Assuming "fs.img" is a byte-for-byte disk image that should be all that's required there. Though I'm not 100% sure that's the "/mnt/onboard" partition, it'll be pretty much the same as what you're trying to do with `loadu` + `nandw` just with less fiddling with tools.
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