I was reading this question, and it sparked an old memory. I had an Amiga 600 a long time ago. And I used to play Secrets of Monkey Island on it, great game. But then I upgraded the computer and installed a hard drive in it. Now Monkey Island claimed there was not enough memory available.
This memory is corroborated by a comment by Jean-François Fabre
you sometimes have to disconnect the second drive if you're short in memory. And sometimes it's not supported by games. But otherwise leave it plugged in – May 28 '20 at 16:19
I've struggling to think why the presence of a hard drive or floppy drive would decrease free memory in an appreciable way. Maybe the underlying kickstarts or whatever allocate buffers and things. But I would suspect that these would be per open file, not per physically present drive.
So why should a hard drive decrease memory available for games and things that don't even need or want the operating system to be running?