I ran into a problem when using grub-install experimentally in what is obviously a foolish way, since I was unable to boot the machine afterwards. I got round the problem, as I shall explain, but I'm still interested to know why the problem arose.
Having added a second hard disk to my CentOS-6.5 server, as an experiment I gave the command grub-install /dev/sdb after checking (with fdisk) that the new disk was indeed sdb, while the disk on which CentOS is running remains sda.
I assumed that this would leave the MBR on sda unchanged, so that I would be able to boot to the current system as before. But I found that this was not the case; on rebooting the machine hung, with a repeating "-" on the screen.
The only way I was able to recover the current system was to use a CentOS Live USB stick I had to install CentOS on a spare partition (sda12).
With that system running I was able to mount the old system (on sdb10) as /mnt, and the boot partition (sdb2) as /mnt/boot, and then give the command grub-install --root-directory=/mnt /dev/sda On re-booting the old system came up.
(I may say I also tried to run grub-install from the USB stick, but was completely unable to do this so as to re-install the old system. Basically, the old disk had become sdb, while the USB stick was sda. So I mounted /dev/sdb10 as /mnt and /dev/sdb2 as /mnt/boot, and gave the command grub-install --root-directory=/mnt /dev/sdb I then removed the USB stick, and tried - unsuccessfully - to re-start the machine.)
But I'm puzzled by my experience, which seems to suggest that running grub-install /dev/sdb actually affects the MBR on sda. Or have I misunderstood in some way?