Why can't I reformat my external hard drive?

Why can't I reformat my external hard drive?

Added (1). My mac book crashed so I transferred all my data onto an external drive (computer repairer took out the hard drive in the computer) but then one day it started working again, but only if the external drive was connected. I bought a new windows laptop (lenovo G50-30) and I can't see the external drive anywhere when plugged in - not even in disk management. I can't see it on my mac either so I can't reformat it to use on my new laptop. Someone help me please! Basic instructions appreciated!

CEF

If your external hard drive is not even being recognized as being existent by any computer you plug it into, it's probably broken. It could also be that the partitions are formatted in a way that the operating system doesn't recognize, so you might need to boot from another operating system temporarily with a Live CD running something more functional like a Linux distribution, then you can wipe the hard drive completely and reformat it.

Guest

It actually is not a good idea so just erase the data instead, Reformatting is not generally ever used der to the risk involved, For whatever reason, users of this site seem to use formatting when they should simply erase unwanted data

Tom and Anna Marie

It is being recognised on my windows laptop in devices and printers and even in device manager but its not recognised as a DISK so I can change it?

CEF