I have a 512mb card formatted to 483MB. I checked the card a week ago and had 135mb of space consumed. Come 4 days later, I had a engine failure, and do not have a log of the event because I was using fat16.
Though the maximum disk storage for fat16 is 2gb, there is a maximum files per folder of 512. I forgot about this limit until today, and though the documentation recommends fat16 for less overhead and faster writes, it does not mention this fact. I have 479 files (including hidden files of which there are none) in the main folder, and would have dropped them into a subfolder had I known.
It has only taken 3-4 months to consume that many files, and I typically drive for 30min+ at a time, and generally only 3 working days out of the week. With more than double what I have consumed the free space, I feel this is a fatal flaw in my implementation.
Another note, I would recommend not stopping your logs until RPM=0 AND speed=0 (or even better, some sort of a countdown parameter if possible). When the engine failed, it may have taken out the crank angle sensor because RPM went to 0 immediately. I had initially thought this was why I was unable to log the event.
I'm pretty frustrated right now, but this is not in any way a slam on the product. This is something that should have been mentioned but I should have also known. In any case, I highly recommend against fat16 unless some sort of fix is put in place to create a subdirectory of logs when file count is > ~400.