Type in "logread"
You will probably see a message like this:
mmc0: new high speed SDHC card at address 59b4
mmcblk: probe of mmc0:59b4 failed with error -28
The firmware doesn't work (yet) with SD cards.
This thread should be merged with https://community.onion.io/topic/1083/omega2-plus-sdcard-not-working