You are right about any make of card being 'duff'. Whilst on holiday last year I bought a 256 Panasonic SD Card from a well known large store and within 2 weeks it developed a fault. The little gold bars looked very scratched which I knew couldn't be due to heavy use as I had only used it twice to store some floppies. I took it back to our local shop and fortunately for me, I spoke with a young man who knew about the use of these cards with our keyboards as he owned a KN7. He confirmed the card was faulty and changed it for another new card there and then. The new card has been no trouble at all. (Hope I'm no speaking too soon as they say). Considering the faulty card was not bought at the local store I was really impressed with their Customer Service.
Best wishes to you and yours.