Heres the scoop... Basically what probably is happening to you is there are about 10 different types of DVD's out there right now. The way I understand it, when Apple first released the DVD-RAM burners they were 2.6 gb single sided, or 5.2 double sided, and the burners are referred to as being "first generation". Then along came the "second generation" of DVD-RAM in Fall of 2000, along with higher capacity 4.7 single sided and 9.4 double sided media. DVD-RAM was intended to burn DATA only, an inexpensive fast way to backup files, especially digital video. As long as we are on the subject, here's a trick I learned...
There is also "Type-1" and "Type-2" DVD-ROM media. The trick is that the Type-2 DVD-RAM disc can be removed from the plastic cassette, and IF you master it correctly (DVD Studio Pro), it will play as a video DVD in any computer that supports DVD format. Unfortunately, consumer TV DVD players do not support DVD-RAM yet. BUT they are re-writeable, so it is excellent practice for mastering without wasting expensive DVD-R discs. DVD-RAM 2.6 gb Type-2 discs run about $20 each, and and should work in the older DVD-RAM burners... peace ...