I have a flashed firmware on my 360 dvd drive, done by my brother months ago. Games run good up to mortal kombat which was the latest game I got (legit copy). My dashboard is 2.0.13146.0. Just bought Dark Souls and had to spend more cash than on normal xbox360 games since it's limited edition (only version sold in game stores atm), went home and it promted to update my system before being able to play. The update stops at around 80% and screen goes black with a big E66 error. I know this has to do with flashed DVD drive but if the updates reflash the dvd drive why does it show E66?? I dont really care owning a modded xbox360 anyway since I dont play backup games, so it wouldnt bother me if I wouldnt be able to flash it anymore. My console is from around 2006. My brother is the only person I know who could flash the DVD drive to stock fw but said he can't do that for weeks and I cant do it on my laptop... HELP !!
I don't think there is much you can do. It probably will keep getting the error until you get it flashed to stock. Just to be sure though, give this update a try. Link