Secondary storage (or external memory) differs from primary storage in that aspect it is not accessible by the CPU directly. The computer typically uses its input/output channels to access secondary storage and transfers the needed data by using intermediate area in primary storage. Secondary storage does not drop the data when the device is powered down-it is non-volatile memory. Per unit, it is normally also an order of magnitude less costly than primary storage. Thus, modern computer systems normally have an order of magnitude more secondary storage than primary storage and data is hold for a longer time there.
In modern computers, hard disk drives are generally used as secondary storage. Time taken to access a particular given byte of information stored on a hard disk is normally a few thousandths of particular second or just milliseconds. But, the time taken to access a given byte of information stored in RAM is measured in billionths of a second, or nanoseconds. It illustrates very important access-time difference which distinguishes solid-state memory from rotating magnetic storage devices: hard disks are typically regarding a million times slower than memory. Rotating optical storage devices, like DVD and CD drives, have even longer access times. Having disk drives, once the disk read/write head reaches the appropriate placement and the data of interest rotates under it then subsequent data on the track are very fast to access. Consequently, in order to hide the initial seek time and rotational latency that data are transferred to and from disks in big contiguous blocks.
When data exist in disk, block access to hide latency offers wish in designing efficient external memory algorithms. Sequential or Block access on disks is orders of magnitude faster than random access, and various sophisticated paradigms have been developed to design efficient algorithms based on the block and sequential access. Another way to reduce the I/O bottleneck is to utilize multiple disks in parallel in order to increase the bandwidth amongst primary and secondary memory.
Some other instance of secondary storage technologies are: flash memory (for instance USB flash drives or keys), punched cards, magnetic tape, paper tape, , standalone RAM disks, and Iomega Zip drives ,floppy disks.