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Jay,
In IIS have you enabled directory browsing for the virtual directory?
If so, you are best to use a URL path in your field not a UNC path, ie: http://servername/... Not \\servername\...
Regards
Anthony
The default flexviewer popup even works fine with UNC.I did not relize that... I dont use UNC paths in web application (just not adviseable) so I never tried and I definetly have not tried to just open a folder. I will have to look into why the standard esri popup handles this and my use of it does not.