I had a similar problem once. Try:
hit Enter. Find the DVD/CD Drives
click the arrow in front of it to open up.
Find your external drive and right click on it and select Properties. There should be a tab for DVD Region. Click on it and then verify that your current region has been selected. (The US is Region 1) If not, select your country then OK. (It will ask you if you are sure, tell it you are.) You only get to change this 4 more times so make sure you choose the correct one.
Reboot when done and see if it works better now.
If not Try Windows Repair all in one
Download it and save it then run it.
You can skip to step 4 or 5 where it gives you the same picture as in the above link.
Make sure only the following item is checked before hitting Start:
Repair CD/DVD Missing/Not Working
Reboot when done and see if that helps.