Right now, you have GRUB in the MBR, and when the system boots, it reads that, and runs GRUB. GRUB then looks to /boot/grub/grub.conf (might be slightly different for your system) to find out what OSes choices it should display. You pick one, and GRUB loads that.
If you are completely replacing the RH install with Mandrake, you need to have the exact same thing happen as above. So, have the Mandy install put GRUB on the MBR. Yes, it's already there, but if the partitions change from the RH set-up, the original GRUB install won't know where to find grub.conf.
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