Use a normal git repo on the usb drive.
Use UTF-8 as encoding for all the files to make sure that you won't get any problems with that
Before you go to your windows machine, make sure that you have checked out the branch that you want to work on.
Use the checked out files and work with them.
Back at your mac, you can commit and push and do whatever you like.
OR
Use your smartphone as usb drive. If you have Android, you can install botbrew on it and use git on your mobile. With that you can do your git operations while at your windows pc.