the number of 100,000 writes per sector is about right. Reading doesn't do anything much. As Jon said, Flash memory is nothing like RAM found in computers. (try fitting a battery into an SD card and you'll know what I mean
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Remember that a format simply wipes the first bit of information in the file system, effectively erasing any reference the file system had to the files on it. The files are still there, just the file system has discarded them. A delete works in a similar way, but has to wipe the reference to each file independantly, so takes longer. Personally, I format. Its quicker and gets rid of everything at once, and its not any more harmful than deleting.
Leo