Install ReadyNAS OS6 on ReadyDATA RDD516

I've acquired a working RDD516 - this is a ReadyDATA model, not a ReadyNAS, so it's running the OpenSolaris type firmware.

I'm told that underneath, the hardware is the same between the RN516 and RDD516, so I want to flash ReadyNAS OS6 onto it.

All my experience is with Pro6 units.  I've upgraded loads of those to OS6.   However in this case, I'm guessing I might need to fix the config files on the hidden firmware drive.  Normally I'd boot into linux from USB, backup the firmware and config file, and then play, knowing I could replace the files if it went wrong.

But in debian, I can't seem to mount /dev/sda1 to get to the files - i guess it's a different format filesystem, even though tools report FAT.

Ideas?

Anyone upgraded an RDD516 from it's funky useless firmware to OS6?