You might be able to get some access to the corrupted image. First, open Disk Utility, and leave it as is. Then open up a Terminal window (with /Applications/Utilities/Terminal). In the terminal window, type this exactly (without the quotes), but don't hit Return yet:
<pre> "hdiutil attach -nomount -noautofsck -readonly "</pre>
Note that there should be a single trailing space at the end. Now open a Finder window and locate your dmg file. Select it, then drag it to the Terminal window. Now hit the Return key.
If all goes well, the name of your dmg file will appear in the left pane of Disk Utility. Directly under it, slightly indented and grayed out, the volume name stored inside that dmg file will appear. You can try selecting that volume and clicking the Mount button. My guess is, it won't work.
At this point, if you can see the volume name in Disk Utility, I think Disk Warrior or Data Rescue will be able to access it, and possibly get some of your data back. If you want to attempt to repair the volume with Disk Warrior, you'll need to eject it (in Disk Utility), then do the hdiutil command again without the -readonly option. I don't think you'll be able to do any repair on it.
I don't have a truncated or corrupted dmg file readily available to try this, so I don't know how far you will get in this procedure.
You'll still be much better off trying to find the original intact dmg file.
