I use a 1D MkIV loaded with Sandisk Extreme memory cards in both CF and SDHC formats. Most of the year I photograph equestrian events, like show-jumping, where I'll shoot off a burst of between three to five frames as they come over a fence, then there might be a break before I shoot again so the buffer never fills and write speed is never an issue. At this time of year with things like Point-to-Point where a number of riders are coming at you one after the other, I have missed a few shots as either the buffer is full or the card is writing fast enough. I'm wondering where exactly the bottleneck is likely to be occurring, I mean can it be resolved by buying cards with a higher MB/s rating? Are CF cards faster than SDHC cards? Or am I simply taking my camera to the extremes of its capabilities? Existing cards have ratings of 60MB/s (CF) and 45MB/s (SDHC).