I have had a genuine retail-packaged, full-price 64GB SanDisk microsd go bad on me before. It wasn't right away, but after a few months using in my phone holding music, it read fine but acted as though it was write protected (it wasn't - you just couldn't delete anything or write to it.) It couldn't be formatted in any device, and even a low-level format wouldn't work. The replacement I bought has worked fine in 2 phones since then.