I bought a Lexar microSDXC card for my GoPro. It has a second row of contacts that other microSD cards don't have. The card came with a USB reader that uses those extra contacts, but it also works fine in other readers and devices like the GoPro.
What are the benefits of this extra row of contacts?
Best Answer
This is a feature of newer cards called the "Ultra High Speed Bus."
It allows the card to transfer data faster.
USB 3.0 is like this too - it has additional pins which allow faster data transfer, but is still backward compatible with things that do not have the additional pins (will fall back to slower speed).
From Secure Digital Wikipedia page:
Without these extra pins you can only do UHS-I which maxes out at 104 MB/s.
Nice chart on the page comparing all the speeds as well: