A:Answer Jake, it is a case of "Can't get there from here". The problem affects ALL files on the MicroSD card except jpg and png images. The problem affects PDFs, Epub, text, and other files. I installed Total Commander and AndroExplorer file managers. They do not see the card.
If I move the filex, using the pre-installed Explorer app, the files will open with any app that I try. If i move the files back to the card, they again become inaccessible.
I have tried swapping cards with my Insignia Flex 8 tablet. The card works fine in the Flex 8 allowing access to all files on the card. The problem is not the files on the card or the apps being used.
This is a firmware problem that should be fixable with an update. I will repeat, the card and the files on the card work OK in an older tablet from the same manufacturer.