203、Hard disk is the preferred type of main storage for most computer systems for three reasons.First, it provides lots of storage capacity.Second, it provides faster access to files than floppy disk drives.Third, a hard disk is economical.Incredibly, a hard disk typically stores millions of times more data than a floppy disk, but a hard disk drive might cost only three times as much as a floppy disk drive.A hard disk is one or more platters and their associated read write heads.A hard disk platter is a flat, rigid disk made of aluminum or glass and coated with magnetic iron oxide particles.You will frequently see the terms “hard disk” and “hard disk drive” used interchangeably.You might also hear the term “fixed disk” used to refer to hard disks.Personal computer hard disk platters are typically 3.5 in diameter - the same size as the circular mylar disk in a floppy.However, the density of the surface particles and the data storage capacity of hard disk far exceed those of a loppy disk.Also, the access time for a hard disk is significantly faster than that for a floppy disk.Hard disk storage capacities of 40 GB and access time of 6 to 11ms are not uncommon.Hard disk drive speed is sometimes measured in revolutions per minute ( ).The faster that a drive spins, the more rapidly it can position the read write head over specific data.For example, a 7,200 rpm drive is able to access data faster than a 5,400 rpm drive.Computer ads typically specify the capacity and access time of a hard disk drive.So “40 GB 8ms HD” means a hard disk drive with 40 gigabyte capacity, and an access time of 8 milliseconds.(4)、When you open a hard disk, you may not see?