It probably means that you Windows 10 has been updated to the "Anniversary Update" (I don't know what the "SO" means). If when you try to look at the device using explorer, you get a message asking you to insert a disk or format the drive, then that is the problem. See
https://www.mobileread.com/forums/sho...d.php?t=276349 for the discussion on this including some workarounds.