I recently flipped over to putting my source code onto my Mac volume, so I could use Time Machine to back it up and immediately got this same problem with my ASP.NET app. Other, procedural applications, built just fine, by the way.
I tried all sorts of things, including using Samba on the Mac side to share the directory, which led into the "too many BIOS commands" error described elsewhere. Unfortunately for me, the Registry hacks to fix that problem never worked for some reason.
I finally found another solution that avoids Samba and just uses the regular Parallels Shared Folders. It too is a Registry hack, but this one simply turns off file change monitoring for ASP.NET. It is a bit heavy-handed, but gets my builds to work again.
The reference for this change is here:
http://support.microsoft.com/kb/911272
The downside to this approach, I am finding, is that you need to be more deliberate about recompiling, or restarting the web server, as changes during development don't just magically appear anymore. I am still deciding whether that is a useful tradeoff.
UPDATE: After several days of this, development was just too difficult and, sadly, what I reverted to was keeping my source inside the Parallels virtual disk. To enable Time Machine backups and Spotlight searches, I used a lightweight MS utility called SyncToy to push stuff out of Parallels and out to my Mac drive several times a day. Despite the high hack factor, it is working well.