I had a problem with the sd card that arrived with my Gen 3 last week but this is now solved. It didn't stop my Gen3 from working but the card did not appear to be recognised.
I thought it was because there were no books on it so I connected it via a card reader to Windows. When I tried to drag some books onto the card Windows gave some cryptic message (can't remember what it was) that it could not do this.
So I formatted the card (FAT), created three folders: eBooks, Music and Pictures and then copied and pasted some ebooks into the ebook folder.
I then removed the card correctly from Windows (using the "safely remove hardware" icon) and inserted it into the Gen3 and switched on.
Perhaps I was lucky because the card has worked perfectly ever since.
