It's what DOS means. Was so named because in the original personal computers there were no hard drives and the OS operated off of a floppy disk instead of a ROM or a tape. See also MS-DOS, PC-DOS, DR-DOS, and FreeDOS.

      In the early seventies, a man named Gary Kildall was experimenting with very small computers that didn't use paper tape.  Instead, they used the early inexpensive magnetic media, which were much faster and had a higher capacity.   Kildall wrote a new OS for machines that could boot off the new type of cheap magnetic media.   His OS, CP/M, eventually became the premier OS for 8-bit microcomputers.

     These were the disks (short for diskette) referred to in the names of the cheap CP/M knock-offs known as MS-DOS and PC-DOS.  Earlier micro- and minicomputer OS'en booted from ROM or slow, expensive paper or magnetic tapes .

     Contrasting the IBM-PC and other microcomputers of the 80s against even earlier micro's, one would expect "Disk Operating System" to refer to hard drives, but then it would have been a Disc Operating System.     ...but that's another argument.

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