The DVD in question is an in-house aerobics instruction guide, and I have been authorised to burn copies.
So I don't really know what I'm doing, but here's what I'm doing.
I rip the DVD using MacTheRipper. I get a folder with a lot of AUDIO_TS and VIDEO_TS files. Then, I open Toast and select said folder. I insert a blank DVD, and hit the big red button. Toast gets to work, the progress bar advances, finishes. The DVD is ejected. Then, when I pop it back in, the DVD won't work.

I've wasted a bunch of blank DVDs in this way.
Any pointers?