I have a Nextbook Ares 11a which is my second one because of this same issue. When I first got the piece of junk it would read the micro sd cards. Then all of a sudden it says they are emply or to reformate them. I know the SD card works, I tested it on other devices. I've tried reformating, repartitioning, etc and this hunk of junk within weeks is doing the same thing. FYI, save your breathe when calling Nextbook. All they know is how to reset/refresh the tablet which I have also completed. The files are just picture on one and music on the other. None work. thanks for your help!
It sounds like there may be an issue with the port where you connect the card. If there is nothing in it (dirt, food, debris, dust, etc.) and no bent parts, then it may have just gone bad and no longer accept cards. Did you try a different card?