The SLRs don't usually use a custom format to store the image short of creating the DCIM/CANON_<number> folder to store the image files. And once you reach a certain limit, they create a new CANON_<number + 1> folder to store additional images.
You should be able to look at all the contents on the SD card in Finder without any problems.
If you can't do that, it might be that the SD card is beginning to fail. What class SD card is it and is it from a well-known brand?
There are a number of reasons that SD cards start to misbehave (I unfortunately have a LOT of knowledge about these thing for work in my previous company):
* An old SD card that is close to the Flash read/write cycles
* A poor quality SD card that used sub-far Flash parts that just failed (or had few read/write cycles)
* Repeated insertions/extractions of the SD card have caused the metal contacts to fail. Trying to clean this up might help
* microSDs use the least reliable kind of Flash memory, miniSDs use a better Flash memory and regular SDs use the most reliable one available. The variation is due to the size limitations of the different SD types.
The specific class of the SD card primarily is used to judge the read/write speeds. But those speeds are possible because "better" internals are used to achieve them. So class 10 SD cards should, theoretically, be better than class 4 or 6 while getting you the expected class 10 speeds.
For really serious people, there is a industrial version of the SD card that is usually 10x more expensive but guarantees life and performance. About 3+ years ago, a 8GB class 10 industrial SD card was about $120 or so..