Outro the drive and remove power and sata for a few min then go back through the guide from the start. If its still happeneing then its Firmware corruption.
You could probably get the key from the dump using HEX but I have never tried and I am not sure if there is a way to tell if the key got corrupted with the rest of it.
Not sure you have a way out other than outro-intro-ing the drive a few times.
RGH can be done to get the key from the board but its a lot of work unless you get someone to do it for you and that will be at a price.
Edit, any hardmods been done to it. Eg cut traces and so on. Would not be the first time!