How to save contents of an edited richtextbox on a C# form

I editted and saved a richtextbox contents that was loaded from a text file using the following codes but it does not work. Anyone can help me.

private void btnSave_Click(object sender, EventArgs e)

rtbData.Enabled = true;

rtbData.SaveFile("c:/DDE_Log1.txt", RichTextBoxStreamType.PlainText);