[Doug Bale]

I use Time Machine and Time Capsule for routine daily backups, but make a SuperDuper! copy every week or so on a second external drive, to have a bootable alternative if my internal drive fails. Last week it did fail, and I copied everything from the SuperDuper! drive onto the new internal, then used the Time Capsule just to retrieve few user files that I knew had been changed since my last SuperDuper! backup.
Now, after a couple of weeks of using the revived internal drive and having verified that it's working properly, I want to copy in the reverse direction, to clone the internal drive onto the SuperDuper! external again. The differences should involve only one small new application and a dozen folders of user files, but this time Super Duper! won't offer me the option of using Smart Update; it wants to completley erase the external drive and start copying from scratch, which will take a day or more.
What's going on?