Hi, i have a macbook pro retina 15" late 2013 model and i want to buy an external dvd drive. I don't use it so much but there are times i need it. I would like to hear some suggestions. I don't want to buy apple's superdrive, because it's too expensive. I need it just to read or write some files and sometimes to watch a movie. I saw some online but many of them have 2 usb ports. Why is that?