Most likely, the reason why the disk in the computer is called "disk" in phrases like "save to disk" even though it isn't a disk is that PCs were popularized at a time when all mass storage media were disks: we had floppy disks, hard disks, and compact disks (CDs), which were all circular and flat like you would expect from a disk.
Example of Disks that Aren't Disks
Before hard disks, there were non-disk-shaped non-volatile mass storage media like tapes, which were rolled by tape drives. Nowadays, we often use non-volatile flash memory, so there are no disks actually involved.
SSDs
For example, you may still see the phrase "save to disk" used when you are saving to an SSD (a solid state drive), even though an SSD contains no actual "disk" in it just like it contains no actual "drive" in it. Both terms come from past technologies.
Although hard disks are still used today, in the past all we had were hard disks. We didn't have SSDs. Nowadays, sometimes a PC only has an SSD and no hard disk, meaning some computers don't have any disks installed inside them at all.
Personally, I think it makes sense to use the term "disk" despite this because otherwise you'll still have to say something, and you'll have to choose one of these:
- Save your files to your mass storage device.
- Save your files to your file system.
- Save your files to your hard disk or SSD, or to your tapes if you have them.
It's much easier to just say "save to disk."
USB Flash Drives
Sometimes you see USB flash drives like M-Systems' "Disk-on-Key," TwinMOS's "Mobile Disk," or EagleTec's "USB Flash Disk," that don't contain any physical disks either but have the word "disk" in their names.
