If you remember my post from a few days ago, you’ll know that I modded my PAL Mega Drive with a region switch. This works well for Mega Drive games, but Mega CD games have their own problems. The Mega CD bios is region locked and will display an error when you start it in the wrong region.
There are a few ways to get other bios roms running on the system. One is to swap the EPROM on the Mega CD with either another region or a region free bios. This is rather invasive and involves soldering. An easier way is to use a Mega Drive flash cart. These can also run the Mega CD bios files, which makes them a good way to make your Mega CD region free.