The information below is now of historical interest only.
New formats such as .dmg have evolved from the Disk Copy format.
See the “HISTORY” section in the yield of “man hdiutil” for an interesting story.
Some, more than others, like to understand about what is going on inside their computers.
I like to understand.
The Disk Copy software by Apple strains my ontology.
Here is what I gather.
A disk image is a file which contains information which is a model of the state of a disk.
Disk Copy is a Mac application that reads and writes such a disk image and deludes most of the system into seeing a real disk that is indeed “mounted” on the Mac even now.
Software designed to be aware of all of the disks attached to a Mac will typically be fooled by this illusion.
Sherlock and the Finder are fooled.
SCSI probe is not.
A file with a name ending in “.smi” is a Mac program that bundles the Disk Copy software and such a disk image state.
Apple distributes Disk Copy as a file called “Disk Copy 6.3.3.smi”.
On that particular imaginary disk there is the real application “Disk Copy”.
It is a figment of its own imagination! A novel solution to the Chicken vs.
Greg Egan would be impressed (booting a self image).
When I fetch software from Apple I see the following pattern.
A .hqx file comes over a TCP circuit (http or ftp), Stuffit turns it into a .sit file and then turns that into a folder with a .smi file.
The .smi file, upon launching becomes a virtual disk holding the same sort of stuff, perhaps, as a distribution CD-ROM.
After running the installer on the virtualdisk I suppose that you can discard the .hqx, .sit, and .smi files.
Of these the .sit is the smallest if you want to retain the ability to retry installation without retransmitting.
Much more info on Disk Copy