I was installing to the larger drive, but since the OS is on the tiny one, the temp folder that the files are extracted to during the installation is also. Anyway I got passed it now, as my edited post mentions. I'm stuck by the general instability of the game itself now though. First I couldn't start missions for some reason, and most recently all the characters were black, and then when I opened the menu to check if I set something wrong and then closed it, it froze again. And this time I can't blame my computer; other than the idiosyncratic drive partitioning, the comp works rather well.