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   Lecture 5

Magnetic Storage 

Read on for an overview of magnetic storage devices like hard disks and floppy disks. 


Hard Disk

Floppy Disks

Taking Care of Magnetic Disks


We know that data stored in primary memory (RAM )is not stored permanently and that we can use a secondary storage device such as a hard disk to permanently store our data.  We can also imagine that a document is stored in computer memory as a series of 0's and 1's.  When this document is sent to be stored on a hard drive, somehow these 0's and 1's have to be stored in an equivalent form on the disk.  The data is not literally stored as a series of 0's and 1's.  Instead the data is stored magnetically.

The same principle applies to a floppy disk since it also stores data magnetically.  The main difference between a floppy disk and a hard disk is in the storage capacity.  Hard drives can store a lot more data.  but then again, floppy disks have the advantage of being portable.

So exactly how do magnetic disks work and how do we minimise the risk of disk failure through careless handling and keep our data safe.

Hard Disk

A hard disk is a sealed unit that contains magnetic platters, read/write heads and other parts. It is sealed to stop dust and fingerprints getting in and ruining the magnetic surfaces inside.

Inside there are a series of platters.  Each platter is coated with a layer iron oxide on the top and bottom surface.  Data is stored on the platters by magnetizing the tiny particles on the surface.  Each particle as given a magnetic orientation; you can imagine some particles being orientated with magnetic north in one direction with other particles orientated with magnetic south in the opposite direction.  

This makes it easy to store an equivalent of a series of 0's and 1's; all we have to do is decide if magnetic north is going to represent a 0 or a 1 and then magnetic south will represent the other number.  

When a series of 0's and 1's are magnetically written to the disk, this information is kept intact until a magnet is used again to re-polarize the particles.  This is why our data is kept safe even when the computer is powered off -  unless you wave a magnet next to you hard drive that is.  I wouldn't recommend doing this unless you want to lose some of your data.

The magnetic charge on a hard drive is gradually lost after about three years.  You would want to refresh your data by recopying it after about a year to be on the safe side.

Let's have a look at a diagram showing what the inside of a hard drive looks like.

What does each part do?

  • Well, this example has three platters that hold the magnetized information.  Platters are 3 " in diameter.  This is the same size as the platter inside a floppy disk (that's why they are called 3 " disks.).  The platters are held on a central spindle and spin at thousands of revolutions a minute.  Typically 5400 rpm or 7600 rpm

  • Each platter has a pair of read/write heads.  The platters have to spin around so that the read/write heads can be positioned over different parts of each platter in order to read or write data.    

  • You would think that data on a hard disk would be written to one spindle first and then another when the first one is full.  This is not the case.  In order to minimize movement of the heads and to make reading and writing faster, data is stored at the same position on each platter before moving to the next sector to store data.

Hard disks have much greater capacity than floppy disks.  Modern hard disks capacities are measured in gigabytes.  Typically 40 to 120 gigabytes.  The greater capacity is partly because of the metal surface, partly because the platters are sealed inside the case and partly because a hard disk has lots of platters, whereas a floppy disk has only one.

Note: --  It is quite possible to have more than one hard disk inside your computer. You can usually attach up to four hard disks to a motherboard.  They are attached in pairs using one cable for each pair.  One will configured as the master drive and one will be the slave.  

You can get more than four hard drives on a computer using an IDE or SCSI card.  I have 6 hard drives on one of my computers.



Floppy Disks

There is one disadvantage of storing everything on a hard disk.  Unless you use a removable drive, the data is not transferable between computers.   This means we need to have other methods of transferring out data - a floppy disk is commonly used for this purpose.

A floppy disk can hold 1.44 Mb of data.   Inside the plastic jacket there is one platter.  The floppy drive on the computer contains a pair of read/write heads; one for each surface of the platter on the floppy disk. (The floppy disk does not actually contain the read/write heads, unlike hard drives.)


Note:- Older floppy disks used to be larger - 5" compared to today's 3.5".  Also the jackets used were not rigid but floppy.  Also, older disks used to be single-sided and not double-sided like modern disks.  This means only one surface of the platter held data.



You can see the platter inside a floppy disk by pulling back the metal shutter. The metal shutter protects the platter from dust and your fingerprints.   When the floppy disk drive is inserted into the drive, the metal shutter on the floppy is pulled back.  When data needs to be read or written to the floppy, the platter is rotated to the correct location for that data.  A platter rotates at 300rpm or 600 rpm.  The plastic jacket is lined inside with fabric to clean the disk as it spins.  This also prevent friction between the platter and the plastic jacket.  Floppy disks don't generally last as long as a hard disk because the platter is not sealed.  

Taking Care of Magnetic Disks

All magnetic disks need to be handled with care.  With careless use data can be lost or worse still the disk can become permanently damaged.  You should take the following precautions so you don't lose your valuable data. 


Caring for Hard Drives: 

  Don't do any of this:-

Don't drop or cause vibration to the drive.

Don't place your computer near a magnetic source like a television set or speakers.

  Do this:-

Defragment you hard disk regularly but not too often

Check your disk for errors regularly using a utility such as scandisk.

~~ Back up ALL your data regularly ~~


Caring for Floppy Disks:  

  Don't do any of this:-

Remove the floppy from the floppy disk drive while the indicator light is on.  This means that the drive is still reading or writing to the disk.

Keep floppies away from heat sources: for example don't leave them lying in the sunlight.

Keep disks away from magnetic sources, like a television set or speakers..

Don't open the metal shutter and touch the platter with your fingers.

Don't bend or squash a disk, say by keeping it in your pocket

Don't press on the disk too hard if you are writing on the label.  Really you should write on the label before sticking it onto the floppy.

  Do this:-

~~ Back up ALL your data regularly ~~


Note: -- Floppy disks can be write protected.  This means you can protect your files from being altered or deleted.  You can do this by clicking down the notch on the corner of the disk.  To reverse this, so that you can write to the floppy, click the tab up so the square hole is covered again



That is folks!!


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