CD/DVD Reader Stopped Working After Yosemite Upgrade

My partner and I both have Mac Mini's (late 2012) versions. They came loaded with Mavericks. He recently upgraded to Yosemite and I did not. I have heard there are "Wi-Fi" issues and "don't want to fix something that ain't broke"!! He connects directly to our ATC, so he doesn't need to be concerned about those issues, however; he came home from getting an Xray yesterday and wanted to copy the CD they gave him. He's never done that before, so I said: "Sure, give me a blank and I'll burn a copy for you. Loaded up the Xray CD on his Mac and it started to read then just quit. The drive did not show up in the sidebar of Finder, like it does in Mavericks.....OK, so let me take the disc into my office and see if I can read. No problems, the disc loaded right up. I copied the files into my "Burn Folder"; loaded up the blank and burned all the files to it; just like it's supposed to. This morning I thought maybe his drive might be bad. We both have the same external USB 3.0 CD/DVD burners. Brought the drive and his cable into my office, plugged it in and it took off working like it was supposed to. So that eliminates the drive and cable. Took it back into his office and plugged it into a "known" good port on his machine, same thing, no work and had to eject the disc with a paper clip in the eject hole. Although his machine seems to address the drive (it will start and spin the disc), nothing shows up in his sidebar or desktop. Help...What's the problem???