Go to the system blade and then memory. Press Y on either the HDD or a MU and then press X,X,Left Bumper, Right Bumper, X,X. Once you do this a blade will pop up saying: This will perform maintenance on your Xbox 360 storage devices and ask if you wish to continue. Unfortunately this will clear all of your game updates and is not a selective process meaning you cannot just un-do one game at a time.
I got this off youtube.
EDIT: I just got all the appropriate achievements using the glitch and i didn't have to clear my chace or reset my HDD.