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 ...
![Stick Out Tongue](https://www.linuxquestions.org/questions/images/smilies/tongue.gif)
), 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