Thank you Vinaysimha! I haven't tested the ubuntu releases yet. I gave the latest release you suggested a try and it hung on boot after the message stating the filesystem was being expanded (I waited several minutes). I reflashed again and this time it booted, but the mmc error remains. Seems odd that I have three of these boards that were purchased within one week of each other and this is the only one that has this issue. They all came with the same release of Linaro pre-installed.
I will try various other releases and see what I get. I would try Android just to see what happens with that OS, but I have no intention of using any OS other than Linaro on the board. I did try installing Windows IoT, but had no luck getting that running.