Here's how:
- Boot from the live CD or live USB, in "Try Ubuntu" mode.
- Determine the partition number of your main partition.
sudo fdisk -l,sudo blkidor GParted (which should already be installed, by default, on the live session) 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!If your main partition is in an LVM, the device will instead be located in/dev/mapper/, most likely,/dev/mapper/{volume}--{os}-rootwhere{volume}is the LVM volume name and{os}is the operating system. Executels /dev/mapperfor the exact name. - Mount your partition:
sudo mount /dev/sda2 /mnt #Replace sda2 with the partition from step 2If you have a separate/boot,/varor/usrpartitions, repeat steps 2 and 3 to mount these partitions to/mnt/boot,/mnt/varand/mnt/usrrespectively. For example,sudo mount /dev/sdXW /mnt/boot sudo mount /dev/sdXY /mnt/var sudo mount /dev/sdXZ /mnt/usrreplacingsdXW,sdXY, andsdXZwith the respective partition numbers. - Bind mount some other necessary stuff:
for i in /sys /proc /run /dev; do sudo mount --bind "$i" "/mnt$i"; done - If Ubuntu is installed in EFI mode (see this answer if you're unsure), use
sudo fdisk -l | grep -i efior GParted to find your EFI partition. It will have a label ofEFI. Mount this partition, replacingsdXYwith the actual partition number for your system:sudo mount /dev/sdXY /mnt/boot/efi chrootinto your Ubuntu install:sudo chroot /mnt- At this point, you're in your install, not the live session, and running as root. Update grub:
update-grubIf you get errors or if going up to step 7 didn't fix your problem, go to step 8. (Otherwise, it is optional.) - Depending on your situation, you might have to reinstall grub:
grub-install /dev/sda update-grub # In order to find and add windows to grub menu. - If Ubuntu is installed in EFI mode, and EFI partition UUID has changed, you may need to update it in
/etc/fstab. Compare it:blkid | grep -i efi grep -i efi /etc/fstabIf current EFI partition UUID (fromblkid) differs from the one in/etc/fstab, update/etc/fstabwith current UUID. - If everything worked without errors, then you're all set:
exit sudo reboot - At this point, you should be able to boot normally.
If you cannot boot normally, and didn't do step 8 because there were no error messages, try again with step 8.
- 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 8 does. Experience helping users in chat has shown that step 8 is sometimes necessary even when no error messages are shown.
