Grub should be installed on another hard disk, so the system can still boot with the primary boot device removed. If this is not completed you will need to have recovery media available to boot from.
1)Launch grub as the root user.
2)From the grub shell run the following commands to install grub onto the disk /dev/sdb

grub> device (hd0) /dev/sdb   # maps /dev/sdb onto the “hd0” label (temporary, lasts until you quit GRUB)
grub> root (hd0,0) # tell GRUB where to find the root of the filesystem (and thus /boot)
grub> setup (hd0) # installs GRUB to hd0 aka. /dev/sdb
grub> quit