picture taken from: http://graphx1.com/graphics/page6.html

Hard Disks

1.  The Physical Contents of a Hard Disk

• Consists of several inflexible, circular disks, called platters, on which information is electronically stored
• Platter: made of aluminum, glass, or ceramic and is coated with a material which allows for information to be magnetically recorded on its surface
• Platters, read/write heads, and the mechanism for moving the heads across the disk's surface are sealed in an airtight case for protection

• Stores information using magnetic patterns
• Can write on a hard disk and read from it
• Undergoes 2-3 formatting steps: 1) Low-level formatting: organizes both sides of each platter into tracks and sectors to define where information will be stored on the disk 2) Partitioning (does not always occur): divides hard drive into separate areas to make it more efficient 3) High-level formatting: used to define the file allocation table (FAT), a table used to locate files
• Multiple platters are stacked on top of one another
• Each platter contains a read/write head for each side
• The hard disk has arms that move the read/write heads to the proper location on the platter
• Due to the stacked nature of the platters, the locations of the read/write heads are often referred to by cylinder (the location of a single track through all platters)
• While the computer is running, the platters rotate at about 3,600 to 7,200 revolutions per minute, and continue to rotate until the computer is turned off
• This spinning creates a small layer of air (two millionths of an inch) between the platter and the read/write head
• The read/write therefore floats above the platter and does not make contact with it

• Step 1: The circuit board controls the movement of the head actuator and a small motor
• Step 2: A small motor spins the platters the entire time the computer is running
• Step 3: When software requests a disk access, the read/write heads move to the FAT to determine the current or new location of the data
• Step 4: The head actuator positions the read/write head arms over the correct location on the platters to read or write the data

picture adapted from: Discovering Computers 2000 (see below)

4.  Advantages and Disadvantages

• Advantages: can purchase hard disks which stay in the computer or which are removable, fast access time (because spinning fast and constantly), continually getting cheaper as technology improves
• Disadvantages: some newer forms of storage can hold more data more efficiently and at a lower cost

• Can typically store from 2 to 16 gigabytes of information
• A typical Hard Drive stores about 8 to 10 gigabytes

6.  Cost per megabyte

picture taken from: http://cgl.microsoft.com/clipgallerylive/default.asp

• Once again, hard drives come in all different sizes
• Hard to price because hard drive is usually included in cost of computer
• A 15.3GB EIDE Hard Drive is sold at Circuit City for $159.99.  Price per megabyte: $.01

This Is A Student Website

Last Edited On May 14, 2000

Information for this page based largely upon:
Shelly, Gary B., et al.  Discovering Computers
    2000: Concepts for a Connected World.
    Cambridge: Course Technology, 1999.