Posted be neodev
There is a problem with certain Megadrive modes where the CD bios replacement doesn't work properly, showing the fixed image.
However, you can force megasd to just do the RAM Cart emulation instead of full cd bios replacement. In order to do so, create a file named the same than the CD bios file you are loading, but ending in .cfg (if you have us_bios_2.00w.bin, name it us_bios_2.00w.cfg) then open with notepad and write
then when loading that bios file as a rom in megasd, it will boot using your original CD bios, and megasd will act as just a RAM cart
I posted this as the op has posted the same question In a different thread where it always nice to have all in one place