get flash360 (Homebrew application for rgh/jtag. google it)
FTP it to your xbox. then run it however you load your games (xexmenu/fsd/etc)
USe the program to make 2 or 3 dumps of your original nand.
Copy those nand dumps to your computer and load them into J-Runner.
Make sure they match by using nand compare in j-runner.
Then you can click the Create ECC button to build a new xell image.
Then put that in the same folder where you got the nand dumps from the xbox (rename to updflash.bin) and run flash360 again. Then use that program to flash the new xell image back to your xbox.
---------- Post added at 22:23 ---------- Previous post was at 22:20 ----------
Where did my edit post button go?
Anyway, if you used this program J-runner to make your rgh nands, then you should already have the newest xell, otherwise, check google for the newest release download links, they should have instructions on how to install it