Tiny bit more info needed i think .
For example the model, and if it needs an external power source.
Some external HDDs need small USB cables so that power won't be lost from the cable's resistance. (Their power needs are just within the cable's capabilities.)
My external HDD is like that. Also, some hdds may need one of those double USB cables that take 2 USB slots in order to give more power.
Is it a new out of the box HDD or did you own it for long?
my external hard drive is not accessible and cannot format. How do i fix it?
If you are on Windows, go to Accessories (under All Programs), right-click on Command Prompt and select Run as administrator, cmd window appears and type this:
chkdsk x: /f
and press Enter.
x = drive letter of your external hard. Before x: and after x: there is one space.