Hi Aymeric !!!
Thanks a lot for your help !!! Even if it would be easier in french, maybe the community would be interested by your knowledge.
I just looked in the bios and you're right, there are two bars missing here. (attachment 1) What is very weird is that in the software of the Asus CPU, it sees the 256 GB well (attachment 2). So I guess we can rule out a simple slot failure. I took the pins off and put them back on but that didn't change anything. For the manipulations, I had found a tutorial that said to go to the system settings to uncheck a "max ram available" box, with no effect. Do you have an idea that I could explore?
Merci à toi et bonne fin de journée,
Martin.