DMG

From Applepedia

DMG is Apple's format for disk images, also called UDIF (Universal Disk Image Format). It's got tons of options including encryption, read-write functionality, sparse images, and so on. You can mount a DMG by double-clicking it in Finder; you can create, burn, and repair DMGs in Disk Utility.

History

DMG is essentially NDIF (Apple's earlier disk image format) with the resource fork flattened into the data fork of the file, so that it wont be corrupted when transferred through the internet. It replaces NDIF for OS X.

Using in OS 9

You'll need "Disk Copy 6.5b11", a little-known leaked beta of Disk Copy.