If you’ve just installed CentOS 6 on software RAID and it won’t boot off /dev/md0, try the following:
1. Using your CentOS install media, boot in to rescue mode
2. Once you are booted in to rescue mode, select the “start shell” option
3. Enter the following command:
chroot /mnt/sysimage
4. Re-install GRUB (/dev/md0 may be different for you depending on how you configured your software RAID)
grub-install /dev/md0
5. Reboot
The above commands helped me fix this issue on CentOS 6.3 x86_64 minimal.
https://www.centosblog.com/fix-for-centos-6-wont-bootload-grub-on-software-raid-devmd0/