On occasion you may find that your disk does not have GRUB installed. Such reasons could be mbr errors or misconfiguration, software raid issues, or any other host of issues. Not to worry, one can re-write the GRUB bootloader onto a disk by doing the following booting a rescue disk or Live CD, depending on the linux distribution being used. Next run the following commands:
sudo grub root (hd0,0) setup (hd0) grub-install /dev/hda
Note you will need to specify whatever disk path that needs GRUB. Now eject the disk and reboot the machine. GRUB should boot up as normal on the disk you wrote it onto.