TFO,
You know that the Hotmaps card will read on your unit but have you used the SD memory card that you are trying to update your unit with successfully with your unit before?
What software version is on your 797c2 Si unit now?
What brand, model and size memory card are you trying to use to update your unit?
Try reformatting the SD card, if it is an SDHC type memory card (4GB or larger) be sure and use the proper reformatting software (
http://www.sdcard.org/consumers/formatter_3/ ). Than download a new copy of the software update file and try it again.
You may have to try another memory card. We recommend the SanDisk brand memory cards.
Now, before actually starting the software update: make sure you have a stable source of power as once the unit starts updating any power interruption could be catastrophic! I would disconnect any accessories or cables other than power, transducer and GPS before making the update as well.