In short, I checked out a DVD from the library yesterday...it had full circle scratches...nearly looked like a vinyl record on the back plus a few smudges...my iMac accepted the DVD & it loaded.
I bought a new DVD set (an old mini-series from the 80s) 3 weeks ago...brand new, no smudges, etc & every time my iMac 27 inch screen took the DVD, it would have a sound like it's trying to find something, the it'll pop out. I tried it on my cheap dvd player near my tv & worked fine. Still can't get it to accept the DVD & load it into the iMac
So, my question is...why would a dirty dvd that has scratch lines around a dvd like a vinyl record accept and load the dvd fine but would spit out a clean dvd after a few weird noises then ejects on its own?
Try it with DVD Player already open, if you haven't done so yet. There are some copy protection schemes (bad sectors, disjointed directories) that newer players can deal with but Apple's OS itself doesn't like (but DVD Player can bypass).
You can also check the console app to see if there are any messages related to the DVD. That might help shed some light on what is going on with this particular DVD.