If SuperDuper ever throws a red "X" at you, go to the Log. See the button at the bottom called "Send to Shirt Pocket..."?
After you do what I suggest below, you'll want to click that button, and do whatever it tells you.
It occurs to me that there is a chance that -- don't freak -- you have data corruption. I'd bet even if you had it, it's not bad, because you have been able to transfer and backup successfully at least twice.
But if you know how to run Disk Utility from your installer discs, that would be wise. Do it on both your internal and external drives. If you have TechTool, DiskWarrior, or Drive Genius, great, run 'em just for kicks.
And if you have anything with the name "Norton" on it, please put it in the nearest garbage pail. Seriously.