#1 - Is your external optical drive a CD drive or a DVD drive? CDs and DVDs are not the same format. A CD drive can only read CD discs. A DVD drive can read both DVD and CD discs. Additionally, Blu-Ray movies requires a Blu-Ray drive which can also read DVD and CD discs.
#2 - You need a program to play back the DVD movie. I prefer using Media Player Classic (Google it). It includes the necessary DVD codec.
Thanks for your help. Yes, it's a DVD multi compact disc rewritable drive so should play both CDs and DVDs. I thought it would just play when I plugged it in, especially as I'd run the small disc that I got with it. Why is it so difficult!! I don't understand all the techno messages I'm afraid, what's a Codec?? I tried that Google website you recommended but I got a message that it could harm my computer so discarded the download. Any other suggestions??? It's soooo frustrating!