Hey there, not sure if you'll still need this after how long it's been but here goes.
First, it's not possible to transfer files between the Xbox 360 & the PC directly with a USB device as the 360 won't recognise the device, you need to purchase a Xbox 360 Transfer Kit - Ebay - and then google for a site called max console and search for this How to Insert GameSaves/Profiles Into Your HDD [PICS] in the forums. I'm not going to link to the site or post as I don't want to get in trouble.
Second, I don't see that it will mess up your game as the game runs from disk and not the 360 itself and as for messing up the console it shouldn't do as it's just a save file and can be deleted if need be, that and loads of people do this all the time especially for 360 games as this will up your gamer score without having to play the game... I don't personally as I don't care what my score is, as you can see in my signature.
Anyway I hope this helps you out, also feel free to add me to your friends list if you'd like.
Last edited by Deadly Stream; 01-21-2008 at 10:45 AM.