Partition with boot flag doesn't boot anymore
I have several linux distributions on my laptop (about eight). Each have their own set of partitions (on an extended partition) for boot, root, home and var (swap and tmp are shared).
I was using a boot partition to run grub2 to show the menu and allow the chainloading to each distro boot partition (with their own grub or lilo), and everything was working ok.
The thing is, after an automatic (big mistake) Ubuntu upgrade (11.10 to 12.04), the Ubuntu boot partition started to show up first. I thought I just needed activate the other partition boot flag, but using GParted or cfdisk, that partition is already marked with the boot flag?? How come a boot partition without the boot flag boots at system restart and the one with the flag doesn't?
Is there other configuration that can mark a partition as the one to boot first?