Install Windows after I’ve installed Ubuntu?
Installing Windows after Ubuntu is not the recommended process for a dual boot Windows and Ubuntu system, but it is possible.
Step 1: Make a partition to install Windows
Resizing your Ubuntu by using gParted (mininum 70Gb is recommended) if necessary and format the the free space as NTFS
Step 2: Boot to the Windows installer and install Windows on the NTFS partition.
Because the Windows bootloader has replaced grub, Windows will automatically boot and you won’t see the grub menu allowing you to choose Ubuntu
Dont worry about it! We got next step
Step 3: Plug Ubuntu installer USB/DVD and run Ubuntu from LiveCD or LiveUSB and install boot-repair.
Open up a terminal by pressing Ctrl+Alt+T and type the following commands:
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update sudo apt-get install -y boot-repair && boot-repair
Step 4: Repair boot
Boot-repair will automatically launch, select recommended repair to repair grub.
Reboot and that’s it. You should now have a Grub menu on boot, allowing you to choose from Ubuntu, memtest, and Windows