FileStreams in aspx page?

[sunburnt]

How can I write data using a filestream during a session? I've tried setting various IIS permissions, but I keep getting

System.UnauthorizedAccessException: Access to the path "C:\inetpub\wwwroot\whatever\data.txt" is denied. at System.IO.__Error.WinIOError(Int32 errorCode, String str) at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean useAsync, String msgPath, Boolean bFromProxy) ...

Here's the relevant code:

private void BtnDotDotDot_Click(object sender, System.EventArgs e)
{
	if (!this.IsPostBack)
		return;

	string sName = Request.QueryString.Get("name");
	if (sName == null)
	{
		// ...
	}
	try
	{
		FileStream fs = new FileStream(Request.PhysicalApplicationPath + "\\data.txt",
				FileMode.Append, FileAccess.Write, FileShare.Write);
		StreamWriter sw = new StreamWriter(fs);
		sw.AutoFlush = true;
		sw.WriteLine("Name:\t" + sName + ":\t...");
		sw.Close();
	}
	catch(Exception ex)
	{
		Response.Write(ex.ToString());
		return;
	}
}