Welcome.
To be honest, the best answer is to pull the power from the Win7 drive. Then boot off the Mint CD/USB and just proceed as normal. This isolates the drives completely - no chance of a screw-up. Once it all looks ok (i.e. after a reboot and update to Mint), plug the power back to the Win7 disk, change the BIOS boot order to the Mint disk and reboot.
From a terminal in Mint (you'll be an ace by then ...
), run this command to update the boot menu so you can boot either Mint or Win7
you'll need to enter your password for this command - it won't print your password