Cards for 5d mk 111
Honestly, although I suppose there is a small risk of corruption, I just use one CF card, and it's fast (UDMA 7) and large (64 gb). The CF slot reads and writes very quickly, but the SD card is much, much slower. Unfortunately, just putting an SD card into the camera makes the whole read/write speed as slow as the SD card - even when writing to the CF card. So, I just avoid the SD card. People made do for a long time with just a CF card. I wish the camera had dual CF card slots intead of an SD slot and a CF slot. But using a really fast, really large CF card only works very well for me.