BIOS version 1.8 is the current version for this board. First, recommend upgrading to that. I have no idea if that will allow CSM or not.
Next
I don’t know what “suggested mode” you are talking about (okay I think I do but that is an assumption). It could be some YouTube video, or someone’s posting. I do my best to not assume, it makes for bad advice.
Now if that post only had been found when searching for “truenas grub unknown filesystem” …
Instead it returned a lot of unresolved posts in the old forum.
Anyhow, its working now, it was just for a test, I dont need it and the only reason I posted was to help.
Thanks for pointing out an actual fix, I am sure this will be helpful for the next person having that issue