E64

it is a hard disk failure, power down remove the hard drive and reseat it, power on and play

You could also try to clear the Cache to do this access 'memory' in the system blade, and then highlight the hard drive. Now press:

Y, X, X, LB, RB, X, X.

The following message should appear: "Do you want to perform maintenance on your Xbox 360 storage devices?"

Accept and the Cache should be Cleared, YOUR SAVES WONT BE AFFECTED, BUT you will have to download again any Game Updates you previously had...

This option is not intended for public use, but ive read it helps on hard drive issues...

USE AT YOUR OWN RISK (even if it isn't really dangerous)...