COMPUTER SCIENCE

             REMOVABLE  HARD DISK                                                                            A type of disk drive system in which hard disks are enclosed in plastic or metal cartridges so that they can be removed like floppy disks. Removable disk drives combine the best aspects of hard and floppy disks. They are nearly as capacious and fast as hard disks and have the portability of floppy disks. Their biggest drawback is that they're relatively expensive.

                       HARD DISK
The term hard is used to describe anything that is permanent or physically exists. In contrast, the term soft refers to concepts, symbols and other intangible and changeable objects.https://www.blogger.com/blogger.g?blogID=7423979372168233121#editor/target=post;postID=4529639920865938934
                                                                                           
                                FLOPPY DISK

I)A soft magnetic disk. It is called floppy because it flops if you wave it (at least, the 5��-inch variety does). Unlike most hard disks, floppy disks (often called floppies or diskettes) are portable, because you can remove them from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less storagecapacity, but they are much less expensive. And most importantly, they are portable.
Floppies come in three basic sizes:

8-inch:The first floppy disk design, invented by IBM in the late 1960s and used in the early 1970s as first a read-only format and then as a read-write format. The typical desktop/laptop computer does not use the 8-inch floppy disk.

5��-inch: The common size for PCs made before 1987 and the predecessor to the 8-inch floppy disk. This type of floppy is generally capable of storing between 100K and 1.2MB (megabytes) of data. The most common sizes are 360K and 1.2MB.

3��-inch: Floppy is something of a misnomer for these disks, as they are encased in a rigid envelope. Despite their small size, microfloppies have a larger storage capacity than their cousins -- from 400K to 1.4MB of data. The most common sizes for PCs are 720K (double-density) and 1.44MB (high-density). Macintoshes support disks of 400K, 800K, and 1.2MB.                          SYSTEM DISK

 Another name for a bootable diskette, a floppy disk (or CD-ROM) that a computer can use to boot the operating system (OS) if the hard drive fails to boot the OS. Typically, PCs will read the floppy disk drive before the hard drive when booting, so if the user leaves a floppy disk in the drive that does not contain the operating system or another control program, the computer will display a system disk error. The user can change the system BIOS so that the hard drive is read before the floppy drive.