I had installed a Ubuntu 12.04 and Fedora 17 dual boot on my system. During the installation I had installed Ubuntu first and Fedora later. Fedora had recognized Ubuntu and added it to the GRUB OS selection list. Afterwards I installed some routine updates on my Ubuntu and after that I am just not able to see the GRUB OS selection anymore when I boot. I am unable to understand what happened, both Fedora and Ubuntu use GRUB 2.0. Also it seems Ubuntu is not able to recognize other existing linux operating systems; because in the beginning I had installed Fedora first and Ubuntu later, but Ubuntu did not recognize Fedora at all, while Fedora recognized Ubuntu when I installed the other way round.