A Mission of Gravity / Hal Clement
The Andromeda Strain / Michael Crichton
Ringworld / Larry Niven
20,000 Leagues Under the Sea / Jules Verne
The Fountains of Paradise / Arthur C. Clarke

I disagree with your conclusion that Asimov's writings is not hard science fiction. The engineering principles of robots are detailed in almost every story (by that I mean both the "Laws of Robotics", and localised developments in his stories). Likewise, the causes and methods of psychohistory are discussed in depth in Foundation, and the characters' interactions with the rules of the science usually stand at the centre of the plot (especially up until "Foundation and Empire", after which too much vague telepathy and mind control additions distort the picture).

Compare that with Ringworld, for instance. Its building mehtods and processes are discussed, but they require Scrith, a material that we do not know is even possible, extremely large-scale transmutation, as well as other miracles. I do not even mention FTL travel, and that is only because FTL travel is only used in Ringworld by visitors and not by natives. Yet Ringworld is clearly hard science fiction. Miven does not need to convince that the technology is true or explain every last bit of it. He merely assumes a limited and well-detailed set of scientific and technological breakthroughs, and goes on to describe what would be possible given those breakthroughs from the technological and socilogical perspectives. That is what makes it hard, and that is what makes Asimov hard, too.