I have partition my harddrive into about 15 different partitions using an extended partition.

I installed many different linux distro's on this harddrive each of the 15 partitions has one os installed on it. I am able to boot into any one of them
At bootup. (i.e grubs menu displays them all and I am able to select any one as I wanted)

My question is each partition has a /boot/grub directory with a menu.lst file
Which file does update-grub write. And if I did a grub-install /dev/sda on that drive where would it look for the menu.lst (i.e in sda1 ,5,6,7,8,...)???

Basically I want to beable to tell grub to boot a specific file on a specific partition. (i.e add it to the menu.lst with out effecting any of the other menu options.....)