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?
On 02/11/2014 01:50 AM, Timothy Murphy wrote:
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.
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a usb install does not properly set up an install. at least now in times i have done them and why i install as explained below.
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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?
if the installer allows the ability to select where the boot loader will be installed, i have found i am better off installing loader to the partition where new systems is going.
there are too many 'gotchas' waiting to get you and grub is a fine example.
grub and maybe grub2 is/are not designed to be able to distinguish between a cd/dvd install and a usb install. if it/they did, a usb install would not assign the usb stick as hda/sda. or so has been my experience with usb installs.
from early times of grub installs, i have continuously installed boot loader to install partition. after install is completed and reboots, i boot into previous hda/sda boot partition.
i then rewrite fstab to show new installed partition and setup a mount directory. ie, /hdd/d/06, mount the newly installed partition, open grub.conf for new install and copy & paste it to grub.conf of hda/sda boot partition. reboot and select new install, then finish second stage of install. if there is one.
a little involved, i agree. i could set old install to chain boot new install, but i am a little set in my ways. ie, i had problems first time i tried chain booting. ;-)
ymmv.
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g -
Timothy Murphy