WCF Service cannot write to network location -- in spite of App Pool Credentials

Created a WCF service that is being called by a Silverlight app. Both reside on the same machine, though the Silverlight app is obviously running the client browser.

I have created a separate Application Pool for the Service in IIS 7.5 and told it to run as a specific domain user that has rights to the network share where we开发者_JAVA技巧 need to create a file. I've tested logging into the specific directory and created a file. However, the call in the WCF service CANNOT create the file.

Incidentally, the user of our application is not an Active Directory account -- they are configured elsewhere... so the Service runs as it's own account in spite of who is logged in.

I've Googled this to no avail as most of the hints are to give rights to the App Pool. Since the geniuses of the programming world hang out here, I'm hoping to get better results. :)

Take a look here. Also triple check what credential is being passed over into the service.