Sometimes dual layer DVDs can suffer what's known by terms such as layer separation, delamination, laser rot and DVD rot. These are all the same thing really. What happens is that the glue holding the layers together can come undone. The result is that air enters the cavity between the layers, oxidizing the metal holding the information, destroying it. The reason this happens in the first place is usually as a result of poor manufacturing, but putting extra stress on the hub of a DVD can accelerate this process.
Sometimes when discs are being pressed small flaws and errors are introduced, which can lead to a disc becoming partially unreadable. Depending on the player and the severity the problems will vary from minor glitches to a disc being totally unplayable.
Both disc defects and layer separation are usually replaced free of cost by the studio. To help identify that that's the real issue however it's recommended to use software such as Nero CD-DVD Speed
to check it first. Do the following:
- start a software DVD player such as PowerDVD and insert the disc you want to test
- press play once so the disc starts, and then stop it. This is to unlock the disc so that CD/DVD Speed can read the encrypted information
- start CD-DVD Speed
- click the Scandisc tab
- click start
- wait until all of the disc has been checked
On a normal disc you shouldn't get any damaged or unreadable areas. Scratches and physical damage can also make the disc unreadable, but will of course not be replaced by the studio. Some discs that use copy protection however will trigger read errors regardless, and in those cases it's hard to tell whether that's really the problem or not. Post the results in the forum and hopefully we'll be able to figure out if that really is the case or not.
One final note of caution - this will only show whether the disc is having readability issues on your specific dvd drive. What that means is that if your drive is going bad disc errors might be an indication of that instead. Try multiple discs in that case.