I'm sure I couldn't find actual totals for everything, but between Sega Genesis, Amiga, Atari ST, and CD32, I was able to count up to about 400GB.
Mostly I looked up how much the TOSEC collections were and added them together.
That's most likely not counting much from Aminet, but I could be wrong.
This isn't including things like X68000 based systems, and I think the Turbo Grafx used a m68k didn't it? I'm sure there are other platforms I'm forgetting.
slaapliedje