I have a MacBook Air running Mac Mountain Lion on one partition, and Ubuntu 12.10 on the other. Whenever it takes me to the ubuntu startup option of:
Ubuntu More Ubuntu Details Mac 32-bit Mac 64-bit
Choosing the Mac 64-bit option just shows a blank screen of the background color of the previous menu. The only way to get to the mac partition is by holding down
option and choosing it there. Then, however, it goes straight to boot and doesn't accept any more hotkeys. And if I try holding down
command-s at the beginning, nothing happens at all.