Corrupted BACKUP .dmg - any way I can retrieve the data?

Question

A friend needs to keep confidential data on a Mac so I suggested using an encrypted sparse image dmg file. So far it seems to be working fine but I had to test it and of course managed to break it.
I created an encrypted sparse image file, added some files to it, and it all worked fine. I backed it up to a USB hard disk and still it worked fine. THEN I added one file to the copy on the USB disk, which still seemed to work fine, and closed it. Now it won't open because Disk Utility tells me that the image file is corrupted. The Finder won't let me copy the file from the USB drive because "some data in the file could not be read".
Anyone know how to fix the file? I'd rather not buy software if I don't absolutely have to since I won't be doing this sequence of events again.
Dave

Answer 1

One really good app to try as well is 'data rescue x' It was a while ago that I used it, and I think they may have changed the name since. It worked like a charm on an ipod of mine that died. I'm pretty sure it was free to try for a time period, but again that was a little while ago.
The interface is a little weird, and when it recovers ffiles they're in a strange heiarchy, but like I said, like a charm.
hope it helps