I have to say that I do not agree about the SD slot being crippled. If the camera has been designed to use UDMA 7 CF cards it should have also been designed to accomodate UHS-1 SD cards. To the best of my knowledge the two specifications came out at about the same time. (Nov 2010 and May 2010 respectively). As Canon built the ability to save file to the two cards into the camera they should have made it a realistic proposition.
For anyone following this thread, and who is interested, I have formatted my UHS-1 SD cards and this makes no discernible difference to the time taken to the number of burst shots achievable or the time taken to clear the buffer (compared with the cards formatted in my laptop).
Not to worry, the 5DIII is still a brilliant camera. Pity I can't use it properly!
That is a reasonable point, but unfortunately Canon did not design it that way. The 5D mk3 does not support the faster UHS-1 (ultra high speed) standard for SD cards, which means that no matter which SD card you use the write speed cannot exceed 20MB/sec. What's worse is that if the 5D mk3 detects the presence of an SD card it sets the default write speed to the slower card - so the write speed of your CF card will be limited too. I shoot a lot of sports, and when I bought by 5d mk3 initially I accepted the idea that I needed two cards - in case there was a problem with one of them or so I could keep a RAW and JPEG copy of each file on separate cards. However, I was continually having problems with the buffer filling and I was not able to keep up with the action. Now I have removed the SD card completely and since then I have not had a problem. In my opinion having an SD clot on this camera is a complete waste of space - they would have done better to make two CF slots available.
I use the sandisk UDMA6 CF card which is rated at 90MB/sec. Never tried a UDMA7 card but at the moment I don't need it.
Not so! The SD slot is useful to have a Eye-Fi card and/or other SD card that has MagicLantern installed
I'm not 100% certain on the timing, but at the design & prototyping time I don't think any UHS-I hardware was available, even for testing. I could be wrong about that though. The other question to ask is, can the DIGIC handle 2 very fast cards at the same time? I'd be willing to bet that the DIGIC doesn't even have enough high-speed data lines to support UHS-1 + CF.