Full implementation of the micro-SD (That's the one that's currently driving the market) requires 8 pins including power and gnd. SD doesn't require the write-protect. Even in SPI mode, the protocol doesn't change when the larger-scale devices are substituted.

With a peak transfer rate (standard) of 25 mega-nybbles (some mfg's claim they can go faster) in "full-speed" (nybble) mode, they're plenty fast for nearly any MCU, and they can be used in the degraded (SPI) single-bit-wide (thereby saving three pins) mode. Some mfg's claim their SPI can operate at 25 Mbps. Maybe that will mate well, and at speed, with SOME MCU's.

There's no reason a file system implementation has to be part of the device interface. Consequently, the device can be polled to "ask" it what its preferences are regarding sector sizes, block sizes, number of sectors per block, etc.

The only issue will be buying the parts. If you're not using 500 units per hour, you probably won't have enough clout to get a manufacturer to "talk" to you. However, since small retailers are able to offer 2GB micro-SD's at approximately $6, in single quantity, including postage, which costs about $1.40US here in the U.S, there surely is a way to obtain supply.

RE