what don't you reflash the whole backup 512 you made back to it...it is after it tried to update right? and then read the nand?
your problem might be it's trying to update the dash and trying to update the game title (TU)....
something along that line....what do you have to loose....
what dash was in it. maybe because you erased what temp info it had cached in it ...
try different dashes was it 2.0.8496.0 the orig dash...also remember make sure you use a cd that can be read(that you know works) and a low memory usb stick...512mb or less....
also try to extract the first 16mb of your nand(from your copy first then try the on board nand) to see what info it shows in flashtool

when you open flash tool you might have to untick 1bl in the keys menu...it worked for 2 peeps already to get the basic info -dvd key
but the basic info is what you want:)
+ i didn't want to say this but m$ had it's odds of a 360 trashing somewhere on a update and had to be sent back to them..
this is why they honor the repair for it...but you popped it open so that deletes that out...lol
just popping out suggestions....