Smart Copy isn't erasing..?

[thom]

I always use SuperDuper before I do a major version upgrade, and it is that time again.

I partitioned an external hard drive so it would match the size of my OS X partition on my laptop. So those two are equal-sized, and I have successfully used SuperDuper to clone one to the other before.

However, this time around, I tried running the copy and it complained that it was out of disk space. After some poking around I realized that on the destination disk, in the Applications folder, there is the 'Install Lion' app, where on the new machine there is the 'Install Mountain Lion' app. And if that's still there, it made me wonder what else was still around.

Now, I could just wipe the partition and do a copy from scratch, or I could run some kind of recursive diff (or a program that puts a nice GUI in front of the result of a recursive diff) and see what's on the destination disk that is no longer on the source disk.

But isn't part of the point of this Smart Update that it would automatically compare all of the files, and erase ones on the destination that weren't on the source?

Or, does it ONLY look at files on the source, and then see whether the file exists on the destination already, then if their checksums match?

Thanks!