The Ondio has no SD/MMC slot, but only an MMC slot. This is a mechanical problem only. The USB bridge can handle both SD and MMC.
Standard SD cards are 2.1mm thick, while standard MMCs are only 1.4mm thick. If you manage to find a so-called Thin-SD card, you can use it in the Ondio. Thin-SD cards have the same thickness as standard MMC cards.
All standard MMC, MMC plus (aka MMC 4.0), and Thin-SD cards up to 4GB should work, although cards >2GB are untested. MMCs don't exist as >4GB, because the protocol doesn't support it.
Jens