How Storage Media Work

Hard drives and their terminologies. We are going to expand on that by explaining how hard drives work, what filing systems are, and several other forms of storage media.

How Hard Drives Work

Hard drives, and most storage media for that matter, work on a platter format.  That means that information is stored on a circular metallic disk.  CD-ROMs, floppy drives, and optical drives all work on this format with some minor variation.  

The information is stored on tracks.  Tracks are to storage media what rings are to trees.  The number of tracks is dependant on the size of the platter, but track 0 is always at the center of the platter.  This allows the computer at boot-up to always know where the boot sector/filing system track is on the hard drive without having to search the drive.    We'll use the example of the hard drive from here on in for simplicity, and explain how it relates to other technologies after.  (A floppy drive is a hard drive with only one platter)

The Physical Properties

A hard drive itself is made up of several glass or aluminum plates mounted on a spindle.  This spindle rotates at between 5400 and 10,400 RPM depending on the motor.  (Faster access requires faster spins, but also requires better read/write equipment)  In order to read from these spinning platters, an actuator arm (Actually, 2 for each platter) is passed across the top or bottom of the platter.  The actuator arm is made up of light-weight material, and most hard drives integrate the electronics for the read-write head into the arm, saving on weight.

The actuator arm is controller by a Voice Coil Servo.  It is called a Voice Coil Servo because the same electromagnet that is used in a speaker is used in the Servo.  These servos allow the actuator arm to be moved from the inner point of the platter to the outside over 50 times per second.  This means that because of the servo and the platter spin, the actuator arm can touch any point on the platter several times per second.  

The Read/Write head is the most important part of the hard drive. These heads are actually very small magnets (Thin-Film inductive magnets) that can either sense or emit a small electromagnetic charge.  A small charge is sent through the arm and down through the head.  When the current passes over a specific bit, it either has a low or high resistance to the sensing current, changing the speed of the sensing current back up the arm.  This tells the drive electronics what that bit is, either a 1 or a 0.  

The write head can actually adjust the charge on the hard drive.  It re-polarizes the specific section of hard drive that it wants to change by sending an electromagnetic signal through the head.  Essentially, it's like a magnet changing the polarity of a piece of metal be adjusting it's north/south polarity.  This process is called Flux Transition.  There are two forms of Flux Transition, called MFM (Modified Frequency Modulation) and RLL (Run Length Limited).  Most current hard drives use the RLL method, which spreads 1 bits farther apart then 0 bits and encoded bytes differently then MFM

CD-ROM Drives and Optical Drives

Other storage media read and write information in somewhat the same way, expect the physical mechanisms they use are different.  In the case of CD-ROM drives, a laser is emitted through a CD-ROM disk, and depending on whether the information is reflected back or dispersed determines the 0 or 1 bit of that read.  CD-R/CD-RWs have the capacity to change this reflection/dispersion by creating the dimples on the CD-ROM through a process known as "burning".  

Optical disks use reflection/dispersion methods as well.  They are re-writable disks, and come in one of two methods; WORM and EO.  Write-Once, Read-Many (WORM) allows disks to be written once, where Erasable Optical (EO) can be written many time.  Optical Disks have lost most of their market to rewrite-able CD-ROM drives.

The other major form of data storage is Tape Drives.  These drives use the same technology that your tape recorder uses, except for the tape being wider than an audio cassette.  Tape drives are really only useful for backing up data, as they are too slow when compared to all the other formats.  Essentially, to find a specific spot on a tape can take minutes, where a hard drive can be there many times within 1 second.  

Most of the original computer did not have floppy or hard drives, and used audio tapes to store their information.  Most used the same cassette player you listened to normal tapes on in order to store and retrieve information.  Tape is the oldest standard method of data storage.

In Conclusion...

I have broken this lesson (Which would have covered RAID and FAT) into one more section due to two short weeks in a row.  I'm sorry if this is a problem for anyone, but the new year has given my two short weeks in a row.  Next week we will be back to normal.