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?
USB 2 standard prescribes for 500mA (2.5w) limit on the maximum a device can draw. While thats sufficient for reading a disc, its not likely to be sufficient for writing. Therefore some drives provide a dual headed cable, where the second cable is used to draw additional current from a second USB port.
There are instances where ports will provide less then 500mA and some if you're lucky that will provide more. The latter is particularly the case on newer machines, where it became clear USB devices and their power requirements had moved on. Your later rMBP will have a USB3 port, so can provide more power, but I'm unsure if it will still limit the output to spec when it detects a USB 2 device has been plugged in.
You should only really need the second cable connected when writing, so assuming this is not going to be often, the easiest thing to do is get an extension which will allow you to connect to the port on the other side of the laptop. If you're writing more often, it may be best to get a drive with an external PSU.