It's all about reed/write speed. A 10x is not necessarily the best card. There is an xda thread somewhere about sd cards tests. It seems the San Disc 4, 8, and 16 Gb class 4 was tops. Those cards were found to excel in speeds the Nook Color operates at. Just search for that. Also, just because they "advertise" it as 10x doesn't mean it actually is. I have studied this subject for my Canon camera and have learned to stay with San Disc for it.