So bassicly I had this problem with my samsung drive that as soon as I tried to update to the new dashboard it gave me E66 and after 3 days of research the only so called "fix" I could find was to downgrade and then update the dashboard then re flash your drive, in my case I had a flashed Lite-on which could update to the new dash and a flashed Samsung which could not so I replaced the samsung drive with the lite on and updated the console then put the samsung drive back into the console.

***PLEASE NOTE YOU NEED A SPARE DRIVE FOR THIS TO WORK***

TUTORIAL
1.Take your xbox apart
2.Replace your flashed drive with your spare drive
3.Update your console
4.Take the spare drive out
5.Plug your flashed drive back into the console

And there you go, you can now play flashed games on your flashed drive without having to take it to a shop to be re-flashed.


*TO ANYONE WHO WANTS TO HATE THIS IS INTENDED FOR THOSE WHO HAVE A SPARE DRIVE SO DON'T HATE IF YOU DON'T*

Hope I helped!