It's possible to transfer via USB, but it requires a USB capable PC by both the 360s you're transferring to/from.
Your stock 360 mem unit can only be used to transfer music/pics/vids/skins, however for about 40 USD, you can get one of these babies.
Neat little thing you sandwich between your console & your HDD. You go 360-Your PC-Thumbdrive-Their PC-360. & it's cheaper than a mem-unit to boot!.
It's one of the reasons internal HDDs suck.
Hope that helps.