Auxiliary Storage Devices

Auxiliary storage devices are one that isn't built into your hardware but is detachable or removable by design. Auxiliary storage is stores data permanently. Secondary memory is also known as auxiliary or supplement memory. It stores large amounts of data.  E.g. Magnetic(tape, disk & drum),floppy disk, pen drive,  CD-ROM,DVD,DVD-RW etc.

Sequential & Direct access devices

The information stored in storage devices can be accessed in two ways

1. Sequential Access
2. Direct Access

1. Sequential Access :

Sequential access means the data is written or read in sequence one after another and can’t go
directly to the desired location without going through all preceding locations.
Example: Magnetic Tape

2. Direct Access:

Direct access devices are the ones in which any piece of data can be retrieved in a non-sequential manner by locating it using the data’s address. The device is called random access because any data from any location can be read or written directly.
Example: Magnetic Disk

Magnetic Tape

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Magnetic tape is the most popular storage medium for large data that are accessed & processed sequentially. It is coated with a magnetic material such as iron oxide or chromium dioxide.

Magnetic tape has the following features.

• Economical
• Long term storage & reusability
• Compact
• Backup data

Advantages

• Their storage capacity is unlimited because many tapes can be used for storing large amounts of data.
• Low cost of data reels & cartridges & high data recording densities, cost per bit of storage is very low. Tapes can be erased & reuse many times.

Disadvantages

• It is a sequential access nature. They are not suitable for random access.
• They must be stored in a dust-free environment because dust can cause tape reading errors.

Magnetic Disk

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Magnetic disks are the most popular direct access storage medium. A magnetic disk is a thin circular plate made of metal & coated with magnetic material. The magnetic disk can also be erased & reused indefinitely. Each disk consists of a number of circles called tracks. Most popular magnetic disks are floppy disk & hard disk. Information is recorded on tracks. This track is divided into sectors. A good example of a magnetic disk is a hard disk.

Advantages

• It supports direct access medium but magnetic tape supports sequential access.
• Magnetic disks are suitable for both online & offline storage of data.
• The data transfer rate for a magnetic disk system is normally higher than a tape system.

Disadvantages

• It is costly.
• Poor people cannot afford this.

Hard disk

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The hard disk is a direct access storage medium. Data is stored in circles called tracks. Each track is divided into sectors. It is a circular path that stores information on these paths.

Types of hard disks

1.Zip disk

• 100 MB
• Disk drive
• Single hard disk

2. Disk pack

• Multiple platter hard disk.
• Separate read & write head

3. Winchester disk

• Two or more hard disk platters.
• Fixed type.

Floppy disk

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Floppy disks are small removable storage devices. It is also known as a floppy diskette.

There are two types of the floppy disk

1. Mini floppy disk
2. Micro floppy disk

Mini floppy disk

• Its size is  5.25”*5.25”
• Its capacity is 1.2 MB.
• Its speed is 720 rpm(round per minute).

Mini floppy disk

• Its size is  5.25”*5.25”
• Its capacity is 1.2 MB.
• Its speed is 720 rpm(round per minute).

Micro floppy disk

• Its size is 3.5”*3.5”
• Its capacity is  1.44 MB
• Its speed is 360 rpm.

Pen drive(Flash memory)

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Flash memory is a memory chip that holds data. It is less expensive. Its capacity are  8 MB,16 MB,32 MB,64 MB,128 MB,256 MB,512 MB,1024 MB(1 GB),2 GB,4 GB,8 GB,16 GB,32 GB,64 GB  etc.

It is a plug & play device that simply plugs into a USB(Universal Serial Bus) port of a computer. The computer detects it automatically as a removable drive.

Optical disk

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The optical disk is the latest new secondary storage medium. It is a random access medium for high capacity secondary storage because it can store extremely large amounts of data in a limited space. An optical disk consists of a thin circular path coated with a thin metal or some other material that is highly reflective. Optical disks are also known as laser disks because they use laser beam technology for data read/write.

The storage capacity of an optical disk= No. of sectors*No. of bytes per sector

5.25” optical disks have 3,30,000 sectors each of 2352 bytes.

Total capacity=3,30,000*2352

= 776*1000000 bytes.

= 776 megabytes.

• The storage capacity of optical disks is 776 MB(Total).
• The formatted capacity is 650 MB.

Types of optical disks

Commonly used optical disks are CD-ROM, WORM(CD-R), CD-RW, DVD, DVD-RW.

Blu-ray Disk:

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▪ Use Blu-ray technology (blue-violet laser) is used to read Blu-ray disks.

▪ Compared to a DVD’s red laser, a blue laser permits more information to be stored at a greater density.
▪ DVD can store 15 GB per layer, a Blu-ray disk can store 25 GB per layer, and dual-layer disks can hold up to 50 GB.