There are a good few examples of how to write simple mass storage drivers out there: the Mac side is fairly simple. I'm away from home, so can't look this up, so I'm not going to go into huge details, but essentially you write a DRVR that provides operations to do block reads and writes, which would presumably talk to your USB chipset somehow (depending on how much of the processing was on the card and how much was on the Mac), you'd add some drives to the drive queue, associate them with your driver, then when a USB mass storage device turns up on the bus you'd mark that drive as essentially having media in it. The OS would then call your driver to control, read and write from the device. Depending on how the card was implemented you could probably almost implement it like a RAM disc with memory-mapped I/O instead of RAM. Dunno. But delving into the SCSI manager would be a recipe for pain that's totally avoidable.