@wardy the information provided by our tech support was not inaccurate - you should be able to change the f/w to any version you like (including backwards).
The nDAC is a fussy customer when it comes to USB hardware - partitions, size, formatting etc.
Where abouts in the world are you?
I could perhaps, organise getting a tried & tested USB update stick sent to you to try?
(Obviously don’t divulge any personal details here).
Regards
Neil.