ReadyNAS RNDU2000 RND-2A Ultra - borked firmware

Nah, it's been standing around for a good while. As for USB recovery, when I try that by holding down the Backup button when starting it, I only get dropped to a "error: can't read filesystem", "grub rescue>" prompt. The USB stick definitely does not have GRUB on it, so it seems something is getting confused in the existing GRUB when trying to read the stick (which is FAT-32/VFAT formatted.)

I was more thinking along the lines of, since I can boot and have control via the serial console, could I overwrite this botched Ubuntu installation with the downloadable firmware from Netgear? How would I go about that - mounting the internal USB memory and overwriting it, dd:ing the firmware file to it, etc?

Of note is that the internal flash is reported as a 125MB (oddly not 128MB) USB stick when booting - even the BIOS screen says there's one USB stick and two 'regular' harddrives plugged in, before I get to GRUB.  (Two USB sticks when i have the external plugged in aswell.)

If I unpack the tarballs (lzcat root.tlz | tar xf -) of the firmware-files, it all adds up to a little over 300MB, so that's clearly not going to fit on the built-in flash. But perhaps on one of the large SATA drives?