You should run resuce Centos option:

# chroot /mnt/sysimage/
# grub-install /dev/sda
/dev/md0 does not have any corresponding BIOS drive
# grub-install --recheck /dev/sda
/dev/md0 does not have any corresponding BIOS drive
#

An error. The md0 mirror, which is composed of sda and sdb, was complicating things. I though maybe I could fudge the device map by adding an entry to /boot/grub/device.map:

(fd0)   /dev/fd0
(hd0)   /dev/sda
(hd1)   /dev/sdb
(hd2)   /dev/md0

# grub-install /dev/sda
The file /boot/grub/stage1 not read correctly.

That was no good either. Finally, I had to run grub manually.

# grub
grub> root (hd0,0)        
root (hd0,0)
 Filesystem type is ext2fs, partition type 0xfd
grub> setup (hd0)
setup (hd0)
 Checking if "/boot/grub/stage1" exists... no
 Checking if "/grub/stage1" exists... yes
 Checking if "/grub/stage2" exists... yes
 Checking if "/grub/e2fs_stage1_5" exists... yes
 Running "embed /grub/e2fs_stage1_5 (hd0)"...  16 sectors are embedded.
succeeded
 Running "install /grub/stage1 (hd0) (hd0)1+16 p (hd0,0)/grub/stage2 /grub/grub.conf"... succeeded
Done.
grub> quit
# reboot