DVDs not mounting properly

I have a couple of DVDs a friend gave me, and I have noticed some interesting things occurring with them. Firstly, I thought the DVD he gave me was bad, as it would not mount in my iMac G4. It’s an HFS or HFS+ disc, so will not mount in a PC either. I then asked him for another copy, which he gave to me. It won’t mount either. Thinking something weird was going on, I tried it in an iMac G5, with a DVD-ROM/CD-RW drive. Mounted first time. I then tried it in another iMac G5, this time with a DVD-RW (specifically, a Pioneer DVR-K04F) drive, and it wouldn’t mount. So, it seems like this DVD will mount in DVD-ROM drives, but not DVD-RW drives. And I cannot make another copy, as the only drives it will mount in are not Recordable drives! Every other disc I have tried in my iMac has worked. I am stumped.