If you are using redhat 9.0, you need to do the following:
Restart with installation disk 1
When the screen comes up asking whether you want
a graphical or text install, type linux rescue
It will search and mount your current filesystem on
/mnt/sysimage
command : chroot /mnt/sysimage
This changes your root to the mount point of your
previous install. then you just do
/sbin/grub-install
You might want to check /etc/grub.conf to make
sure that both OS's are listed
Reboot, and you should be ok
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The above, from post #6 at:
http://www.linuxquestions.org/questions/history/176437