Hi, I'm running a Sandbox update (Shared Users and Applications) on to a 20 Gig partition. All used to be fine in the past, but today it suddenly stops because there's not enough room for GarageBand files. I have to say that I installed all JamPacks out there -- thats a whopping 28+ Gig of stuff.
Here's my recommendation for a future version of SuperDuper! -- to include a script that excludes those files. Under "Edit selected Copy Script" I can see that SuperDuper already sports some scripts like this, for example to "exclude Norton FileSaver files".
Here are the folders that should be excluded:
/Library/Application Support/GarageBand/Instrument Library/Sampler
...was in my case 11.5 GB 
/Library/Audio/Apple Loops
...was in my case 16 GB
Sincerely, Martin.