How can I repair grub? (How to get Ubuntu back after installing Windows?)

Question

I installed Windows 7, which ate Ubuntu's boot file. When starting up the computer, it now goes straight to Windows, without giving me the option of booting Ubuntu.

How can I get Ubuntu back?

Answer 1

When you install Windows, Windows assumes it's the only OS on the machine--or at least it doesn't account for Linux. So, it replaces grub with its own boot loader. What you have to do is replace the Windows boot loader with grub. I've seen various instructions for replacing grub by mucking around with grub commands or some such, but to me the easiest way is to simply chroot into your install and run update-grub. chroot is great because it allows you to work on your actual install, instead of trying to redirect things here and there. It's really clean.

Here's how:

1. Boot from the live CD.
2. Determine the partition number of your main partition. GParted can help you here. I'm going to assume in this answer that it's /dev/sda2, but make sure you use the correct partition number for your system!

3. Mount your partition:

   sudo mount /dev/sda2 /mnt  # make sure that sda2 is correct!
   

4. Bind mount some other necessary stuff:

   for i in /sys /proc /run /dev; do sudo mount --bind "$i" "/mnt$i"; done
   

5. chroot into your Ubuntu install:

   sudo chroot /mnt
   

6. At this point, you're in your install, not the live CD, and running as root. Update grub:

   update-grub
   

   If you get errors, go to step 7. (Otherwise, it is optional.)

7. Depending on your situation, you might have to reinstall grub:

   grub-install /dev/sda
   update-grub # I'm not sure if this is necessary, but it doesn't hurt.
   

8. If everything worked without errors, then you're all set:

   exit
   sudo reboot
   

9. At this point, you should be able to boot normally.

If you cannot boot normally, and didn't do step 7 because there were no error messages, try again with step 7.

• Sometimes giving GRUB2 the correct configuration for your partitions is not enough, and you must actually install it (or reinstall it) to the Master Boot Record, which step 7 does. Experience helping users in chat has shown that step 7 is sometimes necessary even when no error messages are shown.

Answer 2

If you have a Dell PC you also need to be aware of custom software like Dell DataSafe that comes bundled with Win 7. This will overwrite GRUB every time it runs and you'll need to use a Live CD to boot Ubuntu properly. I found that uninstalling Datasafe stopped any GRUB related problems I was having with dual-booting Ubuntu and Windows.

Answer 3

The windows installer doesn't care about other OS in the system. So it writes own code over the master boot record. Its not a problem of windows installer, its intended. If you reinstall, upgrade windows you will face the issue. Fortunately the solution is easy too.

You need to repair the mbr. Do the following

Boot using a live usb/cd of ubuntu. Use boot-repair to fix the problem.

After booting with live usb/cd ,Run following command in terminal,

sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair

Use Recomended Repair.

More info - https://help.ubuntu.com/community/Boot-Repair

Answer 4

I never got in trouble by using these instructions:
https://wiki.ubuntu.com/Grub2#Recover

First of all, you must start your system from a live cd. Then

"METHOD 3 - CHROOT

This method of installation uses the chroot command to gain access to the broken system's files. Once the chroot command is issued, the LiveCD treats the broken system's / as its own. Commands run in a chroot environment will affect the broken systems filesystems and not those of the LiveCD.

1) Boot to the LiveCD Desktop (Ubuntu 9.10 or later). Please note that the Live CD must be the same as the system you are fixing - either 32-bit or 64-bit (if not then the chroot will fail).

2) Open a terminal (Applications > Accessories > Terminal).

3) Determine your normal system partition - (the switch is a lowercase "L")

sudo fdisk -l

If you aren't sure, run

df -Th  

Look for the correct disk size and ext3 or ext4 format.

4) Mount your normal system partition:

Substitute the correct partition: sda1, sdb5, etc.

sudo mount /dev/sdXX /mnt  

Example: sudo mount /dev/sda1 /mnt

5) Only if you have a separate boot partition: sdYY is the /boot partition designation (for example sdb3)

sudo mount /dev/sdYY /mnt/boot 

6) Mount the critical virtual filesystems:

sudo mount --bind /dev  /mnt/dev
sudo mount --bind /dev/pts  /mnt/dev/pts
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys  /mnt/sys 

7) Chroot into your normal system device:

sudo chroot /mnt 

8) If there is no /boot/grub/grub.cfg or it's not correct, create one using

update-grub 

9) Reinstall GRUB 2:

Substitute the correct device - sda, sdb, etc. Do not specify a partition number.

grub-install /dev/sdX 

10) Verify the install (use the correct device, for example sda. Do not specify a partition):

sudo grub-install --recheck /dev/sdX 

11) Exit chroot: CTRL-D on keyboard

12) Unmount virtual filesystems:

sudo umount /mnt/dev/pts
sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt/sys 

13) If you mounted a separate /boot partition:

sudo umount /mnt/boot 

14) Unmount the LiveCD's /usr directory:

sudo umount /mnt/usr 

15) Unmount last device:

sudo umount /mnt 

16) Reboot.

sudo reboot 

"

Answer 5

Just install easyBCD in Windows 7 and do

Add New Entry > Linux/BSD > (select ) Grub2 > (push) Add Entry

Then you can choose Ubuntu on the Windows 7 bootloader to go to Grub2 (previous bootloader).

Answer 6

Boot-Repair worked for me. It's very very easy to use graphical application, you do not need to use the command line, you only have to click a button :)

All the available repair options are described in the Ubuntu documentation and there is a separate page explaining how to start Boot-Repair (by creating a bootable disk or installing it in an existing Ubuntu live disk) and how to use it.

Just boot a Ubuntu live CD, install Boot-Repair and run it.

Answer 7

I think it is a common task, I also have two HDDs, and Ubuntu+Windows. I hope you can repair it with the right way. Try to follow this tutorial. Any questions, ask me!

Answer 8

When GRUB is broken, the user generally does not have access to systems, so repair must be performed from a live-session (live-CD or live-USB).

There are many possible causes to a GRUB break: Windows writing on the MBR, DRM preventing GRUB from installing correctly, installer bug, hardware change... Updating GRUB as proposed initially by Scott is generally not sufficient, reinstalling GRUB as proposed by Marco is more efficient, but still there are various situations requiring other tweaks (adding options to kernel, unhiding GRUB menu, changing GRUB options, choosing the right host architecture...). Other difficulties for repairing GRUB is the use of chroot, and the choice of the right partitions /disks.

All of this has been made easy in a little graphical tool: Boot-Repair. It shall be integrated in Ubuntu 12.04 CD for easier use, but for people needing it now, there are already some distros integrating it: Ubuntu-Secured-Remix (Ubuntu CD integrating Boot-Repair), Boot-Repair-Disk (CD running Boot-Repair at start-up), ...

Hope this helps.