I have a Zenithink C93 tablet. After 6 months of successful use, all of the sudden it will not allow me to download PDF's. When I try, it says no SD card is installed. However, before the problem began it would download PDF's to internal storage with no problems. Now not only will it not download to internal storage, but it doesn't even give me that option and I automatically get a message that there is no SD card installed. However, when I look at storage in my settings, it shows the SD card and how much storage is used and available on it. Anyone have any suggestions on how I can remedy the problem so that I can download PDF files to my tablet again?