I suggest you to install the Debian bootloader on the same disk where you install the Linux system, while keeping disconnected Windows hard drive, so it won’t conflict with it.
Then when Debian and its bootloader are installed on a secondary hard drive you can customize the Debian bootloader adding Windows entry in grub:
Then you need to copy EFI boot from Windows drive and paste in /boot/EFI folder (root access is required, sudo is no sufficient) and then you need to edit the grub configuration file: /etc/grub.d/40_custom for adding Windows entry in grub boot menu. Finally run sudo update grub.
You can give look in here:
Good Luck