I recently got a Dreamcast, installed GDEmu and discovered that MadSheep's SD Card Maker program only runs on Windows. So I wrote my own bash script for extracting TOSEC archives to the SD card and building a GDMenu disc image with pre-cached menu entries:
Enjoy. I believe the script might not work out-of-the-box on Mac due to usage of sed, which I believe has a slightly different syntax between Linux and Mac. Fixing this is left as an exercise for the reader.
PS. Also, hello everyone, my first post here.