A stupid and basic question
 

I've read a few disaster stories in the forums, particularly in regards to how Smart Updates handle erasing files. I know SD differs from CCC, but I just want to be 100% sure that I'm understanding this:

Is it true that during a smart update, nothing will EVER be erased from the SOURCE drive?

If I understand correctly, there may be files erased from the target drive in order to match the state of the source drive, but I just want to be 100% clear that the SOURCE drive is "safe" during a smart copy.

That's correct. Smart Update operates on the destination drive. It does not delete files from the source.