HOW TO: Use the Undocumented Xbox360 Cache Clear Function
An undocumented feature in the latest Xbox360 dashbaord update has surfaced — a cache clear. 
In order to perform this "maintenance" follow the instructions below. (Thanks, Kakarotto)
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. Do you want to continue? Pick yes or no. Voila (it means ‘there you go’ in French before you start asking us where the Voila button is).
*NOTE: This also clears all game software updates. We feel we have to add this, we are not responsible for any damage you do to your console while performing this procedure. Although we do not see how it is in any way possible to damage said console, we know one of you will find a way and write us an irate email. GamersReports - Game News About Games For The Gaming Gamer