As mentioned in most reference books, this movement is in three sections, with the last section mirroring the first. Adagio, BWV 1001, part one, demonstrated this, with an in-depth look at exactly how the sections were interrelated and, perhaps more importantly, when they deviated from one another.
The middle section of the G-minor Adagio has not been written about as much as the other sections. In fact, one author simply commented that the middle section meanders from the key of G minor to the key of C. Nothing could be farther from the truth!
In a sense, the first and last sections are easier to discuss because the form is so obvious. However, this middle section (which has not been written about), contains some of the most remarkable writing in all the Sonatas and Partitas. In only five measures of music, Bach moves from the sublime to being brazen and dazzling. See this video to find out how.