I'm assuming you, or whoever, have NOT dropped the external hard drive. Is it possible someone else dropped it and didn't tell you about that ?

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'..I used disk management console but even there its not shown..'

This isn't showing up ?

When you scroll down in Disk Management, the total space on the drive is shown on the left beside the rectangular box on the right.

Disk x
Removable
the size
Online

If NO.......

If the external hard drive has a 3.5' hard drive inside of it, you must plug the power adapter it came with into the external encloasure, and that power adapter must be working properly.

The USB port you plug the external hard drive into MUST be able to actually supply 500 ma of current.

The external drive may NOT be detected correctly if it's NOT plugged into a USB port built into the case on a laptop, or a USB port built into the mboard on the back of a desktop case

Troubleshooting USB device problems including for flash drives, external drives, external memory card readers.
See Response 1:
http://www.computing.net/answers/ha..
....

In addition to the info there..

For some desktop mboards you can have problems when an external hard drive is plugged into one of a pair of USB ports on the back of the case that are connected to the same USB controller, if there is another device plugged into the other USB port for the pair, because the pair can't actually supply 500 ma per port - they can only supply 500 ma in total. Usually those pairs of ports are one above the other, not beside each other. If you have two devices plugged into such a pair of ports, try plugging one of them in elsewhere on the back of the computer.

For MANY laptop and netbook mboards similar applies. The built in USB ports often cannot actually supply 500ma per port - they supply 500 ma in total for two ports that are close to each other. In that case, the external drive should work fine if it's the ony thing plugged into the USB ports built into the laptop or netbook, or if you have more than two ports, the only thing plugged into two ports close to each other.