A hard disk is a part of a computer. Normal hard disks use magnetism to store information. When hard disks are used, they are available to the operating system. In Microsoft Windows, each hard disk will get a drive letter (starting with C:, A: or B: are reserved for floppy drives). Unix and Linux-like operating systems have a single-rooted directory tree. This means that people who use the computers sometimes do not know where the information is stored.(To be fair, a lot of Windows users don't know where their data is stored as well.)