I know the traditional answer is that if you underrun it, it's only use now is to put your coffee cup on it.
However, I have heard contradictory reports of being able close and cd-r/dvd-r and opening up a second session to use the remaining space. Normally I wouldn't be so stingy about media, but an insanely corrupted file on my HD lead to my computer freezing midway through burning, and the creation of 3 DVD-Rs coasters before I realized what was happening (ouch) Anyway to salvaged them before I use them as an even more expensive coaster? Any spiffy program I can use to resurrect?
Side Note: File was really corrupted. Any program that accessed instantly froze OS, including Nero 1/2 way through burn. Deleted file from compilation.