If you own an ISO, search for the matching GDI set for emulators. Only use the CDI conversion as a last resort for burning coasters.
To play backups on original hardware (or certain emulators like Redream or Flycast with specific settings), you need the proprietary format. convert iso to cdi dreamcast
The Sega Dreamcast was ahead of its time. It was the first console with a built-in 56k modem, a visual memory unit with a screen, and—infamously—a copy protection system that was cracked wide open within months of its launch. If you own an ISO, search for the