This is a completely ignorant suggestion, but... would there be any value in using SPI RAM chips like the 23K256 as a track buffer? You'd only need a few pins and it should be substantially faster than an SD card running at up to 20Mhz. (In particular I could see making use of the sequential mode, in which reads or writes can auto-increment through the entire chip starting from an arbitrary start address.) Using this device you could cache a track on the RAM chip in its raw GCR-encoded form and refresh/flush to SD during the track stepping interval. If my completely off the cuff math is correct it looks to me like you could easily read or write the entire chip several times in 12ms. If you can code a loop that can rapidly step through the RAM chip, un-GCR the data, and shove it out to the SD card in a large block transfer in that period of time then you should be able to handle full-speed blast-a-track writes without breaking a sweat. If that sounds too short...

At its fastest speed the Mac floppy drive turns at about 600 RPM, right? That's 100ms per rotation. Does the Mac really allow only 12ms for the first valid data sector to come up following a track step? The only way it could possibly do that is if the Mac "interleaves" the starting points of tracks in a spiral pattern around the disk, which I suppose is possible, but... what about the points at which the drives' rotational speed changes? Does the Mac not allow for at least one rotation for the drive speed to stabilize? I'm just curious how long you'd *actually* have to flush and refresh your buffer if you went to track-at-a-time instead of sector-at-a-time operation. "12 ms step" sounds to me like the delay per-track to expect when moving the head across multiple tracks, not necessarily the time window for the next data sector.