I tried an interesting hack for file sharing between Parallels and MacOS. This probably isn't supported but I figured I would give it a try. Created a 100M disk in parallels and renamed it disk2.dmg. I am able to mount it in Parallels with XP or in MacOSX. When trying to mount it in both OS's at the same time I consistently crash.
I work as a unix kernel file system hacker, so I say this with some authority. What you're attempting to do just isn't supported well by most file systems. Almost all local file systems expect full local control of the drive / volume / whatever, unless they're expecting a "twin tailed" configuration - and not many people support twin tailed (it's a nightmare). I wouldn't be surprised if it crashed pretty quickly, and corrupted the .DMG in the process. I wouldn't even try having one OS mounting it read-only - that's also difficult for the FS to handle, unless the FS knows it's not allowed to cache. You should have luck alternating who has it mounted, though. That's kind of obvious, but it might help you share.
