From your usb disk after it boots to desktop
Go to a terminal
do sudo fdisk -l
so you can determine the linux partition you want to create grub in.
Then from the command line
Mine is sda5. yours is likely different
$ sudo mount /dev/sda5 /mnt
Now you are in the partition you need to create a grub in.
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
sudo chroot /mnt
After you update grub you should get a list. It should include Windows
#grub-install /dev/sda **mine is sda. yours may be different
You are not done.
Then reboot, remove the USB device. You should see a grub menu with Linux Mint & Windows.
$sudo umount /mnt/sys
$sudo umount /mnt/proc
$sudo umount /mnt/dev
$sudo umount /mnt