This method is a mix of a couple of useful pages:
http://www.centos.org/modules/newbb/viewtopic.php?post_id=16318&topic_id=5503
- Boot into rescue mode (linux rescue)
- chroot the system image:
chroot /mnt/sysimage
- Start grub:
grub --batch --device-map=/boot/grub/device.map --config-file=/boot/grub/grub.conf --no-floppy
- Install grub on both hard drives:
device (hd0) /dev/sda root (hd0,0) setup (hd0) device (hd1) /dev/sdb root (hd1,0) setup (hd1)
- Quit the grub shell and exit the linux rescue shell and the system should reboot successfully.