QuarksLab has a new blog post with an introduction to dumping flash chips:
https://blog.quarkslab.com/flash-dumping-part-i.html
This reminds me of a talk by CryptoMonkey from DEF CON 24:
https://github.com/CryptoMonkey/Conference-Presentations
He re-gave the talk at Black Lodge Research a few months ago. I can’t find it now, but if you’re good at searching, the video of his DEF CON talk is online.
