Re-install bootstrap code (GRUB)

Please perform the following steps:

Boot from CentOS installation disc (for example, CD #1 or DVD).
Type “linux rescue” at the “boot:” prompt.

Mount all filesystems in read-write mode.
Change root to real root (‘/’) on your hard disk:

# chroot /mnt/sysimage

Re-install bootstrap code (GRUB).
If you wish to re-install GRUB to the MBR on a SCSI or SATA disk (/dev/sda):
# grub-install /dev/sda

If you wish to re-install GRUB to the PBR of Partition #2 on a SCSI or SATA disk (/dev/sda2):
# grub-install /dev/sda2

If you wish to re-install GRUB to the MBR on an IDE disk (/dev/hda):
# grub-install /dev/hda

If you wish to re-install GRUB to the MBR on a HP Smart Array disk (/dev/c0d0):
# grub-install /dev/cciss/c0d0

Alternative Recovery from GRUB Issues – Super Grub Disk
Super Grub Disk has a variety of open source boot recovery tools. For CentOS legacy GRUB SuperGRUB works well.

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