Photographer Jeff Cable purchased a couple Canon 5D Mark IIIs recently and discovered that although the camera offers both SD and CF card slots, you should avoid the SD slot if you want maximum shooting speed. He writes,
[…] 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 […] Without UHS [Ultra High Speed] 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
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.
It might not be a big deal for most photographers, but if your line of work requires clearing the camera’s buffer as quickly as possible, it something you might want to be aware of.