external dvd burner not working

Is it FireWire, or USB, and what is the brand and model number? Have you verified the drive is working by testing it on another machine?

Also, some external optical drives require an AC adapter, so if it's not getting power, you may need to use it with the AC, or purchase an AC if it didn't come with one. In addition, many external drives that are powered by USB have two USB connectors, and this is because one USB connector often does not provide enough power, in which case it's necessary to connect them both at once. I believe a USB 2.0 port generally provides enough power for an external device (probably depending on the external device), but I've found that USB 1.1 ports more commonly do not, and thus require both connectors to be attached at once.