I was trying to install an MBR to a disk from which I had deleted it by mistake. (I was unsuccessfully trying to replace lilo with grub, suddenly had 4 mbr files and instead of leaving one, I deleted all).
I finally solved that with Plop Linux, that distro has a special rescue ISO (only 2 MB!)for installing MBR. However, I still had to reinstall the original OS, but at least the disk wasn't lost.
Thanks for your help anyway!