Ok nevermind...
My friend had bought a 250 Gb Xbox 360 Slim that had been already RGHed and didn't want to tell me the truth (he tought that it would make me mad... as I could care about it ^^; ). When I had launched FSD for a tenth time, I saw that the internal HDD got 250 Gb and could tell that something was wrong (it's way more than 4 Gb isn't it ?).
Thanks a lot for trying to help me, your S360NF is indeed flawless because I could update the kernel just fine XD