UPDATED REVIEW 10/7/2018 James in customer service worked with me for several hours and we were able to make everything work, including my 400 GB micro sd card. My suggestion to anyone having issues is to factory reset, do all the updates (both hardware and software) and then insert your sd card and format it to "portable storage" which incidentally is not the "recommended" option. Next I selected for any content able to be stored to the sd card to be stored there. After taking these steps I was able to do the following: 1. Download my kindle books with their accompanying audible narration to the sd card and either read, listen or have immersion reading, 2. Transfer my personal mobi format books to the INTERNAL storage of the kindle fire device itself in the Kindle folder. Per customer service personal books cannot be read from a micro sd card but this was not an issue for me because even after downloading all my kindle apps there was plenty of space for my mobi files. 3. Transfer my personal videos to the sd card (created a Movie folder for them ). I hope this helps!
ORIGINAL REVIEW: Disappointing. My kindle arrived yesterday and customer service is already sending me a replacement. I purchased this new kindle specifically because it claims to support up to a 400 GB compatible micro sd card. The kindle I received would not communicate with the card for kindle ebooks or audible content. This despite the card being confirmed compatible by customer service as well as the kindle screen notification that the card had formatted correctly and was ready for use.
As to the kindle itself, it performed as expected without the micro sd card installed. It is unfortunately not comparable to the Kindle Fire 7 HDX in either screen view or device speed. As others have noted it is disappointing Amazon has chosen not to produce a current version of the 2013 Kindle Fire HDX. I have used mine for 5 years but alas the screen no longer responds reliably. My Kindle Fire 10 is far superior in both aspects, but of course is limited by size and weight as well as accepting a maximum 256 GB micro sd card.
I will update this review upon receipt of the replacement kindle. I am hoping this was just a singular defective device and not a larger issue of the 400 GB micro sd card compatibility. I am curious if anyone else has had this problem.