Greg, are you using how are you trying to update? iFly Connect? If so, first make sure your little switch is in the right place on your SD card. (That may be what you meant by "obvious things have been checked"). Presume so, the problem might be youre SD reader.. Make sure it's SDHC compatible. And you can try blowing out any dust that might be in there if you have an air can or compressor. Or you can try a different reader.
-Walter