As you know, the most common, and probably the most recommended way of dual booting Ubuntu and Windows is to install Windows first and then Ubuntu. But what if you want to install Windows second? … on a computer with Ubuntu/Linux already installed.
This guide assumes that you have Ubuntu already installed on your system, and will guide you through installing Windows 7/8/8.1/10 when you have a working Ubuntu/Linux partition.
The problem here is that the Windows installer tends to overwrite Grub bootloader or whatever making only the Windows installation bootable. But the good news is that your Linux partition is untouched, including the original bootloader and the other Grub configurations. So restoring or recovering the Grub is just a matter of slapping a few commands on the terminal.
Install Windows on a Separate Partition
If your Ubuntu installation occupies the entire hard drive, to be able to create free space for the Windows installation. you may have to shrink an existing partition using a disk utility like GParted. GParted comes preinstalled on Ubuntu LiveCD.
Boot into a Windows installation media and complete with the installation.
Once installed, because of Microsoft’s false assumption that there are no non-Microsoft operating systems you’ll be taken straight into Windows.
Restore / Reinstall Grub 2 with a Ubuntu Live Media
- Now boot into a Ubuntu Live/USB or CD.
- Open a terminal. (Ctrl + Alt + t)
blkidcommands or GParted to identify your Linux root, and boot partitions. You may or may not have a separate boot partition.
In my case
`/dev/sda1` is the Linux boot partition
`/dev/sda2` is the Linux root partition
`/dev/sda3` is the Windows partition
Now we’ll be repairing our old grub bootloader in a chroot environment.
Setup a Chroot Environment
- Mount your root filesystem under `/mnt`
sudo mount /dev/sda2 /mnt/
If you’re using BTRFS (New Linux B-Tree File System) instead of EXT4, the above command should change to
sudo mount -t btrfs -o subvol=@ /dev/sda2 /mnt/
.. the subvolume name `‘@‘` may change
ls /mntto verify that the correct partition is mounted. And it should list the content of your root.
- Mount the boot partition (SKIP if you don’t have a separate boot partition).
sudo mount /dev/sda1 /mnt/boot
- If you have a EFI partition you should mount that also. It is basically a small FAT32 partition around 100mb. Please see the #example case 2
- Bind the directories, so grub can detect other operating systems, like so.
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
Lets chroot into our existing Ubuntu/Linux system on the hard disk.
sudo chroot /mnt
grub-install /dev/sda (specify the disk `/dev/sdX`, not `/dev/sdaX`) grub-install --recheck /dev/sdX exit
Then restart the computer. On next reboot Grub will be the default bootloader and you’ll be presented with a list of operating systems to choose from (Ubuntu and Windows, of course).
Reinstalling Grub in an EFI Based System
Reinstalling Grub bootlaoder in an UEFI based computer is no different from installing Grub on a legacy Bios system. Here we also mount the EFI partition before chrooting, as we’d do with the boot partition.
Example Case #2
Here /dev/sda2 is the EFI partition and /dev/sda5 is the root.
sudo mount -t btrfs -o subvol=@ /dev/sda5 /mnt/
(For EXT4 filesystem: `sudo mount /dev/sda2 /mnt/` )
- Mount efi partition under ‘/mnt/boot/efi’
sudo mount /dev/sda2 /mnt/boot/efi
Forgetting to do so will result in errors being thrown while trying to execute `grub-install`: “/boot/efi does not appear to be the efi partition”
- As shown previously, bind the /dev, /dev/pts, /proc, and /sys directories.
sudo chroot /mnt grub-install exit
In some rare occasions Grub might not detect Windows. In that case just login to Ubuntu (installed one on the disk, not the bootable Live USB/CD) and execute,
sudo os-prober sudo update-grub sudo reboot