Secondary storage differs from primary storage in that it is not directly accessible by the CPU.he computer usually uses its input/output channels to access secondary storage and transfer the desired data to primary storage. Secondary storage is non-volatile.secondary storage also is known as external memory or auxiliary storage.
Secondary memory is permanent. Secondary memory is non-volatile. It is a slow memory. It has a very large capacity.  




1.  Sequential access device

The standard technique for accessing files and devices is that the sequential method. It may be sort of a magnetic tape that rewinds itself when each use. If you wish to listen to the last song of Associate in Nursing album, you have got to quick forward from the start. Now, let's investigate some benefits and drawbacks of sequential access.

  • Magnetic tape

Magnetic tape is the most popular storage medium for large data, which is sequentially accessed and processed. Magnetic tape medium is a plastic ribbon which is usually 1/2 inch or 1/4 inch wide and about 50 to 2400 feet long. Magnetic tape is coated with a magnetizable recording material, such as an iron oxide or chromium dioxide.
Data are recorded on the tape in the form of tiny invisible magnetized and non-magnetized sport on the coated surface of magnetic tape.
                                                  The tape of the magnetic tape storage is divided into the vertical columns called "frame" and rows called " channels or tracks".



             The older tape has 7 tracks, and they used the 6-bit BCD code format for data recording. As in BCD code, the later A is represented on this tape by the code 110001.

                    The first six tracks are used for recording the 6 bits of the BCD code. And the  7th track is a parity bit which is used to detect the error.

Advantage of magnetic tape:-

  • Cost:-  Magnetic tape is one of the cheapest storage media.
  • Storage capacity:- Storage capacity of magnetic tape is very large.
  • Reusable:- We can delete a particular data and store another data at the same place. So it can be reused. 
  • Portability:- magnetic tape is portable. 

Disadvantages of magnetic tape:-

  • Non-flexibility:- Magnetic tape is not flexible.
  • Transmission speed:- Rate of data transfer is slow in magnetic tape
  • Non-human readable:- Data storage in it is not human readable, form, therefore, manual encoding is not possible.

2.  Direct access device

Now, let's look at direct access. When you care about the speed of access and don't want to fast forward through two hours of a movie just to get to the last scene, direct access is your friend. Because modern storage devices such as Blu-Ray and flash drives have much larger sizes, direct access is an important method of accessing data at any location.

  • Magnetic disk
The magnetic disk is the most popular storage medium for direct-access secondary storage. The magnetic disk is the most popular on-line secondary storage device. 

        A magnetic disk is a thin, circular plate/platter made of metal or plastic, which is usually coated on both sides with a magnetizable recording material, such as iron oxide. Data are recorded on the disk in the form of tiny invisible magnetized and non-magnetized sports on the coated surface of the disk.

A standard binary code, usually 8-bits EBCDIC is used for recording data.

                                      





                          For data recording, the surface of the magnetic disk is divided into a number of an invisible concentric circle, called tracks.  The tracks square measure numbered consecutively from outmost to innermost, ranging from zero.

Each track is further subdivided into the sector. Each track is divided into 8 parts which are known as sectors.