Source:
http://segakatana.com/hardware.php?cat=dcdev&id=dclp100
Here's one strange disc which looks similar to the Dreamcast SYSTEM-DISC 2, but is not.
This disc was used by Sega of Japan, to check various parts of the Sega Dreamcast to see if the hardware was working properly. These discs were never released to developers, as they were internals of Sega of Japan.
Here are two of the many screens you can see when you run the disc on the Sega Dreamcast:
