I started form the [Brian Herbert wikipedia article][1]'s ref section. Ther a lot of sources here but here the one I think are the must significant. 

First there's this [archived blog post by Kevin J. Anderson][2] where you could see those disk :

> To the left, I'm posting copies of the actual IBM diskettes containing the files, with the labels "Dune 7 Outline" and "Dune 7 Notes" in Frank Herbert's own handwriting.

![Dune 7 outline disk][3]
![Dune 7 notes disk][4]

And lower on the blog post you could read : 

>When Frank wrote HERETICS and CHAPTERHOUSE he obviously intended to complete the story. He had planned an entire epic that tied his chronicles back through his whole projected history.

>**Frank Herbert wrote a detailed outline for "Dune 7" and he left extensive "Dune 7 notes," as well as stored boxes of his descriptions, epigraphs, chapters, character backgrounds, historical notes -- over a thousand pages worth.**

Second there's [this interview with Brian Herbert][5] 

> Q: Did Frank Herbert leave you any notes to guide you in writing these prequels?

> A: When dad died he was using a yellow highlighter on Heretics of Dune and Chapterhouse Dune. That's all I knew. Then I got a call from an estate attorney who asked me what I wanted to do with two safety deposit boxes of my dad's. I didn't know they existed. So we went down there, and in them were the notes to Dune 7 -- **it was a 30-page outline. So I went up in my attic and found another 1,000 pages of working notes.** As far as filling in these stories, Dad would say for example, "Between Dune and Dune Messiah it's been eleven years and billions of people were killed in the jihad." That's very enticing. So we're staying right on the Frank Herbert thread of his story line. 

  [1]: http://en.wikipedia.org/wiki/Brian_Herbert
  [2]: http://web.archive.org/web/20071012125808/http://dunenovels.com/dune7blog/page21.html
  [3]: https://i.sstatic.net/ASR8D.jpg
  [4]: http://web.archive.org/web/20071012125808im_/http://dunenovels.com/dune7blog/images/dune7N-sm.jpg
  [5]: http://www.filmcritic.com/features/2009/08/brian-herbert-interview/