Two GRUB setup can boot one each of two installs, but not the other, why?

[yavuz]

Hi, 

I have a very strange GRUB problem, that got me totally stumped.

My system has two disk drives, one of which with two Linux installs.
MBR of each disk has been setup from one of the two Linux installs,  
and both to allow booting either Linux:

IDE disk (MBR setup from IDE part2 (Ubuntu 8.04))
Part1: Mint 7 (Ubuntu 9.04 based)
Part2: Ubuntu 8.04

SATA disk (MBR setup from IDE part1 (Mint 7))
Part1-5: swap and user-data partitions

Grub version of both Linux installs is 0.97.

I have following pair of stanzas for both of Linux installs in their respective
/boot/grub/menu.lst:

title           Linux Mint 7 Gloria KDE, kernel 2.6.28-11-generic
#root           (hd1,0)
uuid            3403d691-06fd-4646-840c-8f936fd99ab9
#kernel         /boot/vmlinuz-2.6.28-11-generic root=/dev/sdb1 ro quiet
splash 
kernel          /boot/vmlinuz-2.6.28-11-generic
root=UUID=3403d691-06fd-4646-840c-8f936fd99ab9 ro quiet splash 
initrd          /boot/initrd.img-2.6.28-11-generic

title           Ubuntu 8.04.1, kernel 2.6.24-24-generic (on /dev/sdb2)
#root           (hd1,1)
uuid            9c17f912-7f27-40c4-a8c2-141924762217
kernel          /boot/vmlinuz-2.6.24-24-generic
root=UUID=9c17f912-7f27-40c4-a8c2-141924762217 ro quiet splash 
initrd          /boot/initrd.img-2.6.24-24-generic

I stress again that pair of stanzas in question in both  menu.lst files are 
copies 
of each other. I have confirmed this by diffing them.

I *can* boot from IDE disk's MBR the Ubuntu 8.04 install,
but *cannot* boot the Mint install. 
Opposite is also true, i.e. I *can* boot from SATA disk's MBR the Mint install, 
but *cannot* boot the Ubuntu install.
In both failure cases, I get "Error 15: file not found." 

In other words, I can boot both Linux installs from the MBR that was setup by 
that very install. No problems. However, neither of them can boot the other 
install. 

Since the stanzas are the same, and one MBR boots each install successfully, 
all files must be where they should be, and the error message is bogus. 

So, to switch between these two Linuxes, I am forced to change boot order
from BIOS everytime. Pain in you know where. :-(

I know it should not be necessary but, I even re-setup the SATA MBR after I
added the Ubuntu 8.04 stanza. Needless to say it did not help.

What am I missing?

Thanks in advance.