Yes, you can. (1) Install Windows on your separate drive. During installation, make sure your Ubuntu drive is unplugged to prevent Windows from overwriting GRUB. (2) Once Windows is installed, plug your Ubuntu drive back in. (3) Now, you want GRUB to recognize your new Windows install. (4) Boot into Ubuntu, open a Terminal and update the GRUB configuration: ``` sudo update-grub ``` This command will make GRUB probe for OS installations on your system, and rewrite its configuration files accordingly. (5) On your next reboot, you should see Windows listed as an option in the GRUB menu. Remember to always backup your important data before doing any major changes, like installing a new OS.