Windows 8: List of all available AppBar icons
msdn.microsoft.com
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I agree though, there isn't a obvious replacement.
That symbol has been consistently used as a save icon for decades now. A lot of people know what it means.
I doubt you will be able to find a save icon that works better. I see no point in fighting it. It will fall out of use soon enough anyway (when automatic saving finds more widespread adoption).
If you think about, it works pretty well exactly because no one is using those disks anymore. No one would think that this icon saves something on an actual 3.5″ disk, it’s the concept of saving that’s meant with this icon. Using a HDD icon or an SSD icon (i.e. going the naive update route) would only be potentially confusing. Those icons were and are commonly used to access the file system (and are consequently more likely to be thought of as representing opening something, not saving something).
I think there is a case to be made for sticking with older (but nevertheless in pop culture widespread and formerly popular) symbols. Take the old fashioned telephone receiver on the icon of the phone app of the iPhone. Why not update that and stick a modern phone receiver on there – you know, a smartphone! Let’s stick an iPhone on there! I think it’s pretty obvious that that would be a bad idea.
Today the floppy doesn't represent a physical object anymore. It is literally an icon, a picture that stands for something on its own.
Compare with the pen and eraser icons: there is no physical paper on which to write (or erase) on a computer. In fact one of the primary goals of personal computing has been to rid us of pen and paper. This has not been reached yet, but maybe the day will come when a pen is as foreign to young people as the floppy is today... But I suspect that this stylized image of a pen will be still around then, with its iconing meaning ("Touch here to edit") only reinforced by the disappearance of the physical pen.
It's crazy that we have all this hard drive space and processing power and yet if you forgot to save and the power's out you can lose hours of work. Won't happen to any of Hackernews readers, but probably have bitten my less tech savy friends and family a few times.