I have previously proposed here a SCSI-to-SD
interface; I've only done
 preliminary brainstorming on it, but think it should not be too hard.  The
 SD card interface is very simple, both from a logic and electrical point
 of
 view: if you don't care much about speed it can be done with SPI. (the
 speed
 should be sufficient to keep up with 68k Mac, Amiga and Atari)  All logic
 can probably be done in a single AVR or even PIC chip, if you run that at
 3.3V, the only additional hardware is the SCSI level shifters. 
    After seeing this project 
http://micha.freeshell.org/ramdisk/index.php
I'm very inclined to at least TRY something on this area. Seems simple
enough for trying.