That fingernail sized card would likely be a Micro SD card some cell phones and MP3 players use them. Regular size SD memory cards and the SDHC cards, as well as the older MMC cards from which SD is derived (and the updated MMC+ cards which for all intents and purposes is an SD card clone without the "lock" or read only notch and some extra pins) as all are the same size except for thickness, where the MMC's and MMC+ are thinner. All four types of these cards have a possible storage capacity of 32GB, but memory chips are not dense enough to make that size... yet.

The MMC and SD cards, and MMC+ all use FAT16; the SDHC cards which are spec'd for capacities from 4GB to 32GB uses FAT32, therein lies the incompatibility. An SDHC socket should be capable of using any of these cards; while an SD socket should be capable of using an MMC, MMC+, and SD card, but not SDHC. Older MMC specific slots may have existed at some point years ago, but SD is the most common card slot seen.

While all MMC and SD cards (and MMC+) up to and including 1GB cards are supposed to be "fully compatible" in an SD slot, it turns out that not all devices that take SD cards will take MMC or MMC+ cards. They really are supposed to, but its just a fact that some devices refuse an MMC or MMC+ card, so check you manual and/or with the manufacturer first.

So while MMC and MMC+ card compatibility might be device dependent; on the high end, the 2GB card poses another difficulty for SD's FAT16, so some early devices also may not be 2GB card compatible, again, it depends on how the firmware in the camera or device was written.

I have two different brand digital cameras that say SD cards up to 512MB can be used in the manual. In one camera the 1GB MMC+ card, 1GB SD card, & 2GB SD cards work just fine; while the other camera refuses the 1GB MMC+ card; yet works fine with both 1GB & 2GB SD cards. Weird!

With the 2GB card installed one camera tells me I will get a total of 730 5 mega-pixel size photo's; or 10,472 VGA sized photo's; while the other camera says I'll get 812 5 mega-pixel sized photo's; still, the card manufacturer says 832, so YMMV.

Beyond that, some 4GB SD cards are available, but officially, they are non-standard being that they are way beyond the current spec for SD's FAT16. I don't have any, so I've never tested any. So, 4GB cards may not be recognized as 4GB; or may not be recognized at all; or may not work in some SD card devices; but they may work in SDHC card capable devices.

SD Sockets can also use SD micro hard drives, and other stuff, if the pins are made proprietary and/or the software embedded in the device is written to access the non-SD memory card device, so its rarely used, but it is possible.

-Steven L. Bender, Designer of Vintage Audio Equipment