Ray,
It is very possible that you just have a corrupted SD Card. Try a new one first and then if it works glue the drive door shut (joke!).
If I were you in the circumstances I think I would go for your niece's KN7000. There are many people looking for KN7000 in your neck of the woods who would buy a KN7000 with the SD Card not working. After all, you can still load using the floppy disk. SO you will still get good value for it.
Actually, if I were you I would just keep both of them but I'm a bit of a hoarder lol.