External DVD reader suddenly not working anymore
That it works fine on a Windows PC but not your Mac is puzzling.

Think back to when you know it worked properly. What software have you updated since then? That could be the cause. For example if you went from OSX 10.10.3 to 10.10.4 and that is the only change there may have been a problem with the update.

In that case you may want to try re-applying the latest Combo Updater for your current OS version. If it was an app of some sort you may have to contact the manufacturer to see if they have had similar issues from other customers.