I'm trying to make a copy of my OSX partition so I can reinstall it after I repartition my drive. I have made a .dmg file that's big enough to hold everything. Here's where I get confused: I regularly ...

Well, there is room for X to be told to handle each file a separate way, even though they have the same extension. Assuming the file itself is okay, use the Get Info pane to tell the OS to use Disk ...