Right, I'm aware that HC stands for high capacity, but there seems to be variability between class 6 Sandisk cards. Observe here:

http://ecx.images-amazon.com/images/I/61a6AhQXcEL.jpg

Notice the 3 sandisk Ultra cards on the top right. Card 1 (middle of row) is a class 6, claiming 20mb/s, however the one adjacent to it on the right is a class 6 claiming 30mb/s (notice the I, or roman numeral 1 next to the SDHC logo), and the one next to that is a class 10 with the same speed rating.

Here's what I found regarding the uppercase i

http://www.engadget.com/2010/06/24/sd-cards-branded-with-an-upper-case-i-are-faster-yo/

but then I find this https://www.sdcard.org/consumers/speed/

Which states UHS and speed class are not compatible, but the sandisk cards lack the UHS-1 class logo (1 inside a U)

I'm just really confused to what speed my card really is