Ok. I DON'T UNDERSTAND what went wrong or right or whatev but.....SOLVED.
Yep. Solved. Just like that. Maybe making sure the BIOS mystery "unlocked" it without me noticing, but the Memory Card saves.....kinda works now. Except when i save, i don't see it saving. Seeing no differences, i thought it still didn't work.
Yet, this morning (yeah, two weeks later.....the situation made me kinda mad. Plus, got shittons of things to do), i got two "clean" Grandia isos and put them in my Rasp 3. Turned out it couldn't save and change disks, or so i thought. Of COURSE it wouldn't! Ha!
And then, i thought i could maybe try to do a Clean Start, saving as soon as possible right at the start of the game, in Parn. In a hurry, i chose "Load Game" instead of "New game". And oh, what do you know, THERE WAS A SAVE FILE! And telling me i was on the wrong disc!
So, i ran the Disc 2, and....the save was there. As if it was waiting since forever for me to realize i was trying to find a solution to a problem that didn't even exist in the first DAMN F.....LYING PLACE!
So, my impressions on this? Having clean, recognized bios, without necessarily clean isos (still running "modified" ones, for the voices...) still makes the Memory Card save files stealthy, yet they're there when you load them. I don't know if that madness will be there on all Multi Disc games, but well.....that's how it is.
Anyway, thanks for all the replies. It was nice to have help and i'm pretty sure the BIOS mystery was the key of this. Even if it wasn't, i wasn't sailing alone on those awful seas, so, many thanks!