[dnanian]

A block copy copies things well below the file level, at the physical block level of the drive. This means that the drive is copied pretty much exactly as-is, including corrupted folders, fragmentation, temporary files, VM swap files, etc.

Every time you copy this way, you copy the whole drive. It takes quite a lot of time: well more than your typical smart update. On top of that, you have to boot from some other drive. But the advantage is that things that operate below the file system or are dependant on low-level file system details (e.g. copy protected applications that lock to your drive) don't (usually) need to be reactivated.

People use Disk Utility to restore when they've backed up to something that's not bootable. A SuperDuper! copy is a volume just like any other, and can be restored by Disk Utility: that's one of SD!'s advantages.

As far as partition sizes go, SD! doesn't care unless things are way too full, as you read. I think that Disk Utility does different types of low-level copies depending on whether or not the volumes are the same size, but I'm not sure.