Hello. I am the Tech Lord from VideoHelp.com.
How old is the disc? Is it from a good quality brand?
If it is very old, and was not treated well, the disc could be rotting. Don't worry: You have time to back it up.
The rotten part has apparently not reached the data surface. Even if it did, error correction could probably compensate for some errors to a limited degree.
Try using tools such as gddrescue or dvdisaster or IsoBuster for Windows and try extracting the same damaged disc to an image file from multiple disc drives.
BluRay discs have several disadvantages compared to the unfortunately discontinued HD-DVD, mostly associated with the fragile position of the BluRay data layer.
*Data layer not in middle of disc (600µm) like on DVD, but only 100µm away from reflective surface! Superior read/write accuracy, but inferior robustness. That's why a good BD must have a very scratch-resistant reflective surface. CDs have the same problem on the other side: only 100µm away from labeled surface. Sony's fragile CD-Rs from 2009 could even be damaged by water on the label side. Sony's DVDs were always pretty good, even before AccuCore. I remember them from my DV-RW250 (2004) in early childhood.
*No BD-RAM exists, unlike HD-DVD-RAM.
In my opinion, both should exist and be supported. The hidden advantages of HD-DVD are just unknown to the public.
I hope I could help you. #AskTheTechLord.
[I hope, that the HVD will have dual-axis redundant data mirroring and will be released soon.]