System.IO.IOException: The device is not ready

private Log()
       {
           if (AppConfigs.IsLogEnabled)
           {
               if (WebLog == null)
               {
                   engine = new CommonEngine();

                   string appPath = string.Empty;
                   if (!string.IsNullOrEmpty(AppConfigs.LogPath))
                       appPath = AppConfigs.LogPath;
                   else
                       appPath = Path.Combine(Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location), "logs");

                   DirectoryInfo di = new DirectoryInfo(appPath);
                   if (!di.Exists)
                       di.Create();

                   string fileName = AppConfigs.ApplicationName + "_" + engine.GetDateSuffix("") + ".log";

                   FileInfo fi = new FileInfo(Path.Combine(appPath, fileName));

                   if (fi.Exists)
                       tw = File.AppendText(fi.FullName);
                   else
                       tw = new StreamWriter(fi.FullName);

                   sb = new StringBuilder();
                   HasError = false;

               }

           }
       }