managing a multiboot without stomping the toes of other OSes

[Rustom Mody]

Hi.

I have a system with ubuntu on sda5 and debian on sda6 and a dedicated
grub partition on sda3 (mbr points to sda3)
The sda3 grub.cfg only chainloads to sda5 with title ubuntu and
likewise sda6 with title debian.
Further bootsector grubs in sda5 and 6 manage the detailed linux options.

The problem is that whenever some package is updated (eg grub-pc) it
'detects' other OSes and makes a grub.cfg with all kinds of junk such
as the debian kernel with the ubuntu filesystem etc etc.  What is
worse, it meddles with the sda3/mbr grub

So what I want is that I should be able to tell the Debian grub:
1. You are sitting on sda6 (not sda)
2. So please leave sda alone
3. Please dont 'detect' other OSes

Likewise in ubuntu

Thanks

Rusi