RHEL: Reinstalling Boot Loader on the Master Boot Record (MBR)

# Tested on RHEL 5 & 6

# In many cases, the GRUB boot loader can mistakenly be deleted, corrupted or replaced.
# We can manually reinstall GRUB on the master boot record, using either "grub-install"
# or the "grub" prompt.

# Ensure the device.map file located in the /boot/grub directory lists all boot devices
# in the same order detected by system ROM.

# grub-install -----------------------------------------------------------------------------

cat device.map
   # this device map was generated by anaconda
   (hd0)     /dev/cciss/c0d0

# Use the 'grub-install' command pointing to the root device.

grub-install /dev/cciss/c0d0


#  --- OR ---


# "grub" utility ---------------------------------------------------------------------------

# Manually set up the MBR and /boot partition from the grub prompt. (hd0) and (hd0,0) should
# coincide with the results from the "find" command. It may be that "find" command will
# fail showing an error message; if that's the case we can continue with the procedure as
# "find" step is not mandatory

grub
   grub> find /boot/grub/stage1
   grub> root (hd0,0)
   grub> setup (hd0)

# ------------------------------------------------------------------------------------------


# If a crash occurred on the server and GRUB boot loader was lost, this may be recovered by
# booting from a rescue CD.

# After the command prompt is displayed, change the root directory to the OS installation
# by typing this command:

chroot </mnt/sysimage>

# Review the /boot/grub/grub.conf file, as additional entries may be needed for GRUB to control
# additional operating systems.