I am using a 2GB SanDisk MicroSD Card and have tried two different cards.
Have you tried putting those cards in another computer, formatting them, copying 2GB of data to them, and then making sure you can read them back?
It's probably not your problem, but there are a lot of counterfeit cards out there. SanDisk is especially susceptible to the unscrupulous counterfeiters because they are such a widely known name. These counterfeit cards often don't work properly, or actually have less memory than stated. These are often sold as "upgraded" cards meaning that they are really smaller cards that have been fudged to report that they contain more memory than is really there. They start out working OK, so people think that all is well. But as the card fills up, it suddenly starts giving them trouble once it reaches the point where there is no more actual memory.
I'm not saying this is your problem. But since the disk expander sketch puts the Linux boot partition at the end of the card's space, problems would show up quickly if it were an "upgraded" counterfeit.
Put the cards in another computer, reformat them, and give them a good workout. If the cards don't work well in the other computer, they won't have a chance of working on the Yun.