I can do the unmount, but I still get the error message when I try to sfdisk. It says it's "meaningless". If I try to do the next step, I get "permission denied" even though I am root.

How big is your card? if it is bigger than some size, you substitute numeral 6 with 'C', to change partition from fat16 to fat32.

It's 2 gigs. I had to reformat it earlier, so I don't know if I'm still having problems with it (I shouldn't be). It's formatted as FAT right now.