I had the same problem and discovered it was a limitation of the codec used. In particular, the DivX 6.4 codecs have a known issue with .WMV format files that are larger than 2.5G or have long filenames. Also be sure that you are not running another instance of a media codec while encoding the project (i.e. opening a streaming file that encodes in the background using the same codec). You may also want to burn the project to your harddrive first so you don't risk your blank discs and waste your time if the burn fails. There are several other issues regarding other converting file formats using bundled codecs that are also bundled in other installed software on your system. I have several versions of DivX installed and they all installed in unique folders and didn't warn me about existing copies. You can see the list of issues at the DivX Support site as well as many posts at this forum. DivX offers several tips and workarounds for other codecs as well. Good luck

Hello McCallum,
Which type of media do you use?
I import 3.65GB mpeg2 to PDR5 and multi trim this file to 2.5 GB.
No problem for burning.