I installed Debian 6 on the same PC where I am running different versions of Ubuntu. Debian 6 installed grub, probably a different version than Ubuntu. I am running update-grub from Ubuntu 10, 11 & 12 and it does not overwrite the Debian boot loader. The Debian grub has failed to correctly define the boot options for 10.04 Ubuntu distro, the other Ubuntu distros where correctly defined. I had to modify the boot options at the grub prompt, intead of single, I wrote recovery nomodeset and I could load 10.04 Ubuntu but not in the recovery mode.