I’ve got two separate drives. Linux Mint on an SSD and Windows 10 on an older, mechanical drive. Leave the Windows drive alone. Make the Linux drive the first drive in your BIOS boot order, with the option to boot to Windows as your second drive.
If your GRUB menu doesn’t show the Windows drive yet, run “sudo update-grub” to detect it. When your reboot, the bootloader should show both options.