sizes?
 

I'm a little confused. My hard disk (iMac 10.11.6) shows, in the Finder - GetInfo, that the size "used" is 88.4 GB. I do a SD clone, and the size of that clone looks to be 76.1 GB. Given that discrepancy, how do I assess how large a backup disk I need for my clone? What happened to the 12.3 GB? Does SD do some kind of compression?

No, no compression; as indicated in "What's going to happen?", we don't copy some files (eg swap files, sleep images, temporary files, etc). Those can be large.

Your backup disk should, in general, be the size of the source drive in total.

Thank you. Wow, swap files, sleep images, temporary files, etc. can really take up a lot of room. I had no idea.

A related question, which I guess isn't related to SuperDuper, but may be related to this answer is why, on my Mac, in the Finder:Get Info, the "Used" area of my disk is 88.4 GB, but when I add up the folders listed in the Finder, the number is a lot less. I have to assume that the folders listed by the Finder don't include swap files, sleep images, temporary files, etc. which are hidden. Is that the case?

There are lots of folders you can't see in Finder...

Though evidently Finder:Get Info reports on the hidden stuff, while the Finder listings do not.

On the volume, you mean?

I mean the Finder:Get info on the volume itself sees everything. That "Used" number is big. That number must include hidden files. When you open the volume and Finder makes a list of the content including "Size" of each, not all is there. That is, the sum of all the "Size" numbers doesn't equal the "Used" number of the volume.

Yes, that's generally returning the size of the volume rather than folders your selecting. But these size things aren't always clear, especially with APFS.

Well, I'm still on 10.11, so I'm still using HFS.

Even so - there are hidden things in HFS+, strange folders that have emulated links and the like, etc... :-)