FileStream -> MemoryStream -> FileStream

FileStream fs = File.OpenRead(filePath);

MemoryStream ms = new MemoryStream();
ms.SetLength(fs.Length);

fs.Read(ms.GetBuffer(), 0, (int)ms.Length);
fs.Close();
			

fs = File.OpenWrite(filePath + ".bak");
ms.WriteTo(fs);

fs.Close();
ms.Close();
fs = null; ms = null;