One notable shortcoming of Microsoft's existing Xbox 360 transfer cable is bullet-pointed out right on the order page: "Cannot be used to transfer data between two Xbox 360 S hard drives."
When the svelte Xbox 360 S was brand new, this was hardly a problem at all; everyone was too busy not having their console die repeatedly to even notice this soon-to-be shortcoming. And now, while your S may not have given up the digi-ghost just yet, it's entirely plausible that many of you traded up for the 320GB Gears of War 3 model and, in so doing, have discovered the above bullet point for yourself. You're in luck, however, since Microsoft's Major Nelson writes, "We have a new cable (pictured) that will handle Xbox 360 S to Xbox 360 S transfers. Head over to xbox.com/transfer to see how you can order the proper transfer cable."
The bad news: Xbox.com hasn't been updated with this purported information as of publishing time, so you'll have to take Major Nelson's proclamation as an early alert or, better yet, follow his advice. "Call support if you have any questions or you want to order from them to make sure you're getting the cable you need." Or if you just want someone to talk to – we get it.