To grant ASP.NET write access to a file, right-click the file in Explorer, choose "Properties" and select the Security tab. Click "Add" to add the "{machinename}\ASPNET" user. Highlight the ASP.NET account, and check the Write box in the Allow column. "
 

 

If you did the above, and you still get the error, it probably means the application is NOT running under the <machinename>_ASPNET user. Look in your web.config for <identity --- attributes and try granting read/write access to the IUSR<machinename> account and / or any other accounts (or groups) the app may be running under.