Do it from a computer, has to be formatted to fat32.
Mount the sd card on your computer, right click, click format, select fat32, UNCHECK quick format.
***make sure you are formatting the card, not the internal storage, this will erase all your data, so copy the contents to your computer if there is anything you want to keep***
***the unroot process also completely factory resets your phone so make sure you have all the data off your phone backed up***