Common Characteristics of Magnetic Storage Devices

All magnetic storage devices have the following characteristics in common:

• Read/write heads use controlled electrical pulses to affect the magnetic structure of the media. Each disk surface has one read/write head, whereas double-sided media use two read/write heads (one per side).

  

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• Hard drives use one or more double-sided platters formed from rigid materials such as aluminum or glass that are coated with a durable magnetic surface.

• All disk-based magnetic media is divided up into 512-byte areas called sectors .

• Sectors are organized in concentric circles from the edge of the media inward toward the middle of the platter. These concentric circles are called tracks .

• All magnetic storage devices require that media be formatted before it can be used. Formatting means that a logical magnetic pattern must be placed on the disk to enable data to be organized and located. Most media is preformatted at the factory.

• All disk-based magnetic media store information about where files are located. The method used to organize this information is called the file system .

• The file system used by Windows 9x/Me uses a special area near the outer edge of the first side of the media to store a record of where files are located; this is called the File Allocation Table (FAT ). Two copies of the FAT are updated whenever a file is created, changed, or deleted.

• FAT-based storage is also supported by Windows NT/2000/XP, but these operating systems normally use a more sophisticated and secure file system called NTFS . NTFS stores information about each file in the Master File Table (MFT ).

• A storage device that emulates magnetic storage, such as an optical drive , requires software drivers that will make it appear as an ordinary drive to the system.

• Disk-based magnetic media uses random access (any sector can be accessed at any time) whereas most tape media uses sequential access (the tape must be read from the beginning to locate data). Some advanced tape drives use special techniques to speed access to data, but the data is still read sequentially.