None of what you say makes logical sense. I'll just tell you some of the things I ran in to.
1. Made a change to an existing template. It started failing but not at the section I made a change to. Had to completely make a new one. (This happens a lot with us.)
2. I believe in 9.5 there was a change in a SP update that caused all of our templates to fail at inject script because there was either the wrong partition named c: or there was no c: volume letter. Had to ad the mount partition.
Have you upgraded your service pack to get ready for the 9.6 upgrade?
Take out the Mount partition and put a wait in your template before the inject script and see what your "only 1 partition" is labeled. Note: from what I could tell you only have 1 partition.