|The external optical drive requires that the USB port you plug it into can supply 500ma, and even if it can it may not work properly in all USB ports - e.g. a port in an external hub.|
There are lots of laptops that can't supply 500ma per port when more than one USB device is plugged into the ports built into it - it can only supply 500ma total for all the ports, or for a pair of ports - if you plug in only the external optical drive it may work fine, or you may need to plug it into two USB ports via a USB Y cord.
External hard drives also require 500ma.
Troubleshooting USB device problems including for flash drives, external drives, memory cards.
See Response 1: