The SD manufacturers' group doesn't seem to want people who use fewer than 100k units per week to use their products. They want you to buy their spec and pay for the updates. If you can justify that, then you're good to go.

SD is easy if you don't mind going very slowly. However, since you pay for the "high performance" whether you use it or not, you should be able to use the 4-bit mode. However, the 4-bit mode requires a fair amount of interface-specific hardware. You can easily implement the interface in programmable logic (e.g. FPGA) but getting the proper specification is a pain.

As an intermediate step, perhaps you'd consider using CF in "true-IDE" mode. That can go pretty fast and is well specified. You have to implement the IDE interface anyway, so attaching a CF module (via a commercially available adapter costing, perhaps, $5US, should be pretty straightforward.

A single-bit-mode SD interface can operate at up to 25 Mbits/second. If that's fast enough for you, perhaps that will be easier yet, as it is essentially an SPI.

RE