Parece haver um bug que mexe com vários sistemas de inicialização.
De lifehacker- the-best- melhorias-em-ubuntu-1004-lucid-lynx :
boot options for non-Ubuntu partitions can stop working. The fix is to run sudo update-grub from a terminal, or after hitting Alt+F2 for a prompt, and Ubuntu is working on "re-spinning" the ISO images that are affected.