Well...after some testing I have determined that, if you care at all about high speed shooting or clearing you buffer quickly, YOU DO NOT want to put a card in the SD slot. Why? Because, for some reason unbeknownst to me, Canon decided to build the 5D Mark III with one very fast CF slot which supports the newer UDMA7 protocol and a standard SD card slot which does NOT support the high speed standard (called UHS - for Ultra High Speed). This is really strange because many other cameras have come out with UHS1 compatible slots over the last year. Without UHS support, the top speed that can be achieved by the SD card is 133x. This is true even if you purchase a 600x SD card and insert it in the camera. The best you will get is 133x
At this point, you might be thinking, "why would the SD card slow down all of the data transfer of the camera including the CF card?" It turns out that the camera will default to the slowest card inserted. So, if you have a 1000x CF card in slot one and any SD card in the second slot, the very best buffer clear that will achieve is 133x. When shooting sports or any type of images with burst mode (6 frames per second), this is crippling. I want to shoot a bunch of images, have the camera clear the buffer as quickly as possible, and then keeping shooting more. Why would I want to clear data at 20MB per second when I could be transferring at 90MB per second or better? For this reason, I almost never use the SD slot in the camera. I want to take full advantage of my Lexar 1000x Professional CompactFlash card.
One more thing. Most of the time, this is a hardware limitation and can not be solved with a firmware upgrade. Even more of a disappointment!
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