This is a quick image based on an analogy which I'm sure others have thought of. But here I try and show the conceptual difference between a computer's hard drive and a computer's memory.
And this applies to any computer. All the way from your mobile phone to the gigantic building sized computers used for predicting the weather.
I anyone wants a supped up one for a poster, let me know and I put in some shading and enable the poster function.