Save
#Save PersonallyPersonally, I am familiar with the floppy disks, and I have used them in the old days, but now, when I see the floppy-disk icon, I don't think of a floppy-disk, I think of the "Save" function. See this answer to a related question on UX.
Floppy disk
The floppy image is well known symbol for save, and even though new generations don't know what a floppy disk is, they do know that the symbol means "save".
Hard-disk
A modern data storage like a hard-disk is not so visually recognizable ( ), so it would not be so useful as an icon. It is a piece of hardware that is hidden inside the computer box.
Memory storage
And now, some computer have started coming without a hard-disk, they come with only large memory storage, using the same technology as we have in memory sticks etc.
Future...
And who knows what the future will bring...