The size of the SD card is not an issue. Back in 2007 we came out with a software update that allows our units to work with the newer SDHC cards (that go up to 32GB). Since the 798 units did not come out until after 2007 they and all other units produced since than have this same software update. Also there have been others that have used the 2GB and larger SDHC type memory cards and have not had a failure.
The problem you had may have been do to a corrupt downloaded file or even something else on the SD memory card. Whatever it was it was not present when you tried again with a different memory card. Lad to hear that the second memory card worked. Did you download the software update directly from the Humminbird web site to this memory card or did you just copy the update from the file you used with the larger memory card?