[dnanian]

No, we don't know how much space the backup is going to take. We know how much it's going to take at a maximum. But, the script can select out one or two files, or more, or whatever, and we don't know what it's going to do without running it, which involves scanning the entire drive. Does that make more sense?

Regarding compression, I truly have no idea. It's all Apple's code, and it does what it does. It's going to completely depend on what you've got in the source.

SD! main focus is twofold. Real "Backups" -- i.e. "Backup - all files" are absolutely designed to deal with mechanical failure. They're bootable, when stored on a supported boot device, and are easily restored in part or in full.

The Safety Clone/Sandbox, on the other hand, is designed to isolate your OS from unwanted changes during software installs (including OS upgrades). By running from the Safety Clone, you can easily boot back to the original OS without losing changes to your real data. But they're not backups, at all, and wouldn't help to recover from any sort of physical damage.