Summarize:

Nero is notoriously inefficient at burning dual-layer video DVDs, because it does not handle the layer break well. This is the spot in the disc where the reader moves from the first layer to the second layer. You will need a program more capable of handling this correctly, such as ImgBurn. In order to burn a dual-layer video DVD, you will need your video file on your computer formatted into the VIDEO_TS folder format. This occurs when you rip a DVD, or copy an unprotected DVD from the disc to your hard drive. Click the Mode menu, and then select Build. Click the small "Browse for a Folder" button to open the file browser. Navigate to your VIDEO_TS folder and click OK. This will ensure that the video will fit onto the disc when you burn it. This will open the layer break selector. ImgBurn will rate all of the available options. Choose the option with the highest rating to ensure the smoothest possible transition between layers. The burn process will begin. Dual-layer DVDs can take a long time to burn, so you may have to wait awhile. Once the process is complete, watch the video in a DVD player to test it out.
Download a different burning program. Open ImgBurn. Click the calculator button. Click the Build button.