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Does anybody have a suggestion for how to move the root partition to a new drive and set up grub2 to boot on that drive? I seem to have no luck instructing grub-mkconfig what it is I want to do (e.g. chroot'int into my new root just confuses all the scripts).

Background I am running Debian Squeeze on a headless low-power NAS. My current setup is / on sda0 and /boot on sde0 (a CF card): I needed the separate /boot because sd[a-d] need to do a delayed spin-up. Now I've found an old 2.5" IDE disk to use as / including /boot to allow me to spin all the big disks down.

What I've tried Basically I went

mount -o rw /dev/sdf5 /mnt/newroot
cp -ax / /mnt/newroot
cp -ax /boot /mnt/newroot/boot

Then I tried 

chroot /mnt/newroot
update-grub

But that failed with grub asking if root was mounted. Then I did a half-hearted attempt at setting up /mnt/newroot/grub/grub.cfg to find the kernel image on sdf5, followed by a grub-install --root-directory=/mnt/newroot /dev/sdf. But this just landed me a grub rescue prompt when I tried booting from sdf.

My backup plan is to just reinstall, so a bonus question (no checkmarks for this one): What do I have to do to get my lvm2 and mdadm config across? Is it all stored in the filesystems (and will it be automatically discovered), or do I need to take of it myself?

Solution (thanks to Maciej Piechotka): As Maciej points out, I need to to a proper chroot for all the grub tools to work. For reference, this is how I did it:

janus@nasguld:/mnt/newroot$ sudo cp -ax / /mnt/newroot
janus@nasguld:/mnt/newroot$ sudo cp -ax /boot /mnt/newroot

All the files are now copied (see here for a discussion of copy strategies). Fix the new etc/fstab to point to new root:

janus@nasguld:/mnt/newroot$ diff -u etc/fstab.old  etc/fstab
    -UUID=399b6a6d-c067-4caf-bb3e-85317d66cf46 /     ext3 errors=remount-ro         0 1
    -UUID=b394b614-a977-4860-bbd5-7862d2b7e02a /boot ext3 defaults                  0 2
    +UUID=b9d62595-e95c-45b1-8a46-2c0b37fcf153 /     ext3 noatime,errors=remount-ro 0 1

Finally, mount dev,sys and proc to the new root and chroot:

janus@nasguld:/mnt/newroot$ sudo mount -o bind /dev /mnt/newroot/dev
janus@nasguld:/mnt/newroot$ sudo mount -t proc none  /mnt/newroot/proc
janus@nasguld:/mnt/newroot$ sudo mount -t sysfs none /mnt/newroot/sys
janus@nasguld:/mnt/newroot$ sudo parted /dev/sdb set 5 boot on 
janus@nasguld:/mnt/newroot$ sudo chroot .

We are now chrooted to the future root exactly as it will look. According to Maciej, it should be ok to just call grub-install, but I did an update-grub first to get a look at the generated /boot/grub/grub.cfg before installing the bootloader. I am not sure it will be automatically updated?

root@nasguld:/# update-grub
root@nasguld:/# grub-install /dev/sdb
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  • Post the contents of /etc/default/grub, and the exact transcript from update-grub.
    – Gilles 'SO- stop being evil'
    Jan 1, 2011 at 17:00
  • (thoughts after adding solution): It might actually be that the problem was that I hadn't updated /etc/fstab in the chroot (this would be consistent with update-grub complaining that "\ isn't mounted"). Were I to do this again, I would try first without bothering to mount the special file systems in the chroot.
    – Janus
    Jan 3, 2011 at 1:12
  • just a note: do not forget to unmount /mnt/newroot before reboot!
    – Giacomo Tesio
    May 16, 2013 at 20:31
  • Here are instructions on how to move your root partition / to new nvme drive, while keeping the /home on hdd. lucasmanual.com/blog/…
    – Lucas
    Sep 17, 2018 at 2:51

3 Answers 3

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Mount basic filesystems and copy/modify files while chrooting like:

  • /dev (mount -o bind /dev/ /path/to/chroot/dev)
  • /proc (mount -t proc none /path/to/chroot/proc)
  • /sys (mount -t sysfs none /path/to/chroot/sys)

IIRC that worked for me while installing Grub 2 in arch and numerous times on Gentoo. Then after chroot to /path/to/chroot command was simply:

grub-install /dev/<boot_disk>

As of lvm2 (and I belive madm but I haven't used it) the configuration is stored on disk. There is configuration what should be read to discover devices. Assuming your devices are in standard locations (/dev/sd* or /dev/hd*) there should be no problem.

PS. I would not trust simple cp of live system as there are several places where it can go wrong:

  • Forgot to change /etc/fstab and other useful files
  • Files changed during access
  • Coping garbage (/tmp etc.)
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  • Thanks. So basically, you are saying I do a sloppy chroot :) I'll give it a try with a proper chroot. cp -ax should be fine (maybe after dropping to single user runlevel) according to this old howto: tldp.org/HOWTO/Hard-Disk-Upgrade/copy.html
    – Janus
    Jan 2, 2011 at 14:52
  • Worked a charm! Thank you very much for the help.
    – Janus
    Jan 2, 2011 at 15:50
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    @Janus: I'd copy data from Live distro to be on safe side. I'm glad however it worked.
    – Maciej Piechotka
    Jan 2, 2011 at 20:37
  • 2
    I had to do update-grub before install-grub.
    – Aryeh Leib Taurog
    Aug 8, 2013 at 17:08
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you can install grub from live distro without chrooting:

grub-install /dev/hda --root-directory=/mnt/guest/
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  • Thanks. I'm not sure this would work here: as I understand it, grub-install only updates the grub image files and writes the MBR: In particular, grub.cfg is not updated. As I commented above, mounting the special dirs might be overkill, but I still think the chroot is the way to update grub.cfg in a simple way?
    – Janus
    Jan 4, 2011 at 7:23
  • Yes, this didn't work for me either. You have to get update-grub to work and that one does not have --root-directory or something, no?
    – Mitar
    Jun 30, 2017 at 6:40
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BTW if you are adding a partition (like a windows) on which you don't want grub to write a in the boot sector, but you want grub to know about it when your computer boots, you can re-scan the partitions and generate a new grub.cfg file by using the grub_mkconfig command as follows in a terminal session

cd /boot/grub

sudo cp grub.cfg ./grub.cfg.old

sudo grub_mkconfig -o ./grub.cfg

Now when you boot off of your current linux partition (that had grub booting it) it will now know about the other partition.

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