The GDEMU ODE for the Sega Dreamcast communicates with the console using 3.3V logic levels. This is a problem with the VA0 Dreamcast consoles as the data bus in this revision communicates with the GD-ROM using 5V. If you connect 5V data lines to the GDEMU, you will destroy the FPGA on the GDEMU in short amount of time.
Traditionally, the fix was to replace series resistors on the data bus going to the GD-ROM drive connector, with the hope of reducing the voltage levels. But this solution doesn't really reduce the voltage from 5V down to 3.3V. Using resistors is not true logic level conversion the way it's properly done in other devices (like Everdrive multi-carts) where proper 3.3V to 5V level converters are used. The GDEMU FPGA data lines still run outside its recommended limit (At about 4-4.5V)
The only real, proper, solution is to lower the entire data bus from 5V to 3.3V. Luckily, that bus (known as the G1 bus in the Dreamcast) has only 3 devices on it: GD-ROM, BIOS and Flash memory. A talented French modder documented the process of how to do this correctly here. The process involves replacing the BIOS and Flash memory with 3.3V parts and lower the entire G1 bus to 3.3V logic. In essence, you are converting the G1 data bus from VA0 to VA1 specification.