Why is the hard disk called a "hard" disk? Why are floppy disks called "soft" disks? Why is the U disk called "U" disk?

The names of hard disks and floppy disks come from the media they use. Hard disk records data in rigid materials, such as aluminum and glass (hard), while floppy disk records data in thin plastic sheets (flexible and soft), hence the name. The name of the USB flash drive is not uniform and has not been recognized. There are various names, called USB flash drive, which should come from using USB interface.