First off, you’re not going to get 20gb of data onto anything smaller than a 25gb blu ray blank BD-R. That much won’t fit onto a blank dvd.
So choose sections to burn to the disk. You’ll get about 4.3gb on a regular, single layer dvd, and about 7.9gb on a double layer dvd.
Finally, turn off “Test write”. Though I don’t have your program, I believe it is simulating the burn, not doing the actual burn to disk.
Edit: Ok, 2gb is much better.