Sometime you find your external harddisk doing something wrong by displaying folder that can’t be opened, instead it asks you to format your drive. it means there is logical failure in the harddisk. Windows by default has provide us the ability to scan and fix disk error. Usually if windows reads flash/external disk that may be corrupted it will show us question whether we want to scan and fix disk or not. In my case, i found some folders unaccessible and windows shows me the question to scan and fix each time i plug in the external harddisk. but when i follow the instruction to scan and fix harddisk the operation is never get done.

thats why i tried to use some other software to solve my problem, i just used simple, free, and low size software, but it didnt solve my problem. so i back to the very basic chkdsk, utility provided by windows to check disk and fix it using command line. but when i read some articles, they said using chkdsk to fix logical error by running a fix utility on harddisk can very easily make recoverable data unrecoverable.

well i thought it will not cause any problem so i run this chkdsk utility
Here is how to repair you harddisk using chkdsk
before you start to chkdsk first locate the drive name on windows explorer for example J: or D:
after that you can do this
1. start > all program > accessories > cmd
2. right click cmd. choose run as administrator
3. run this command
chkdsk drivename /f
if you have your external harddisk on J: type
chkdsk J: /f
4. scanning and fixing process should run, sometime it askd you confirmation about what you will do with you damaged folders/files

u can change the /f option with other option like -p or -r

after the process finished i got my external harddisk working well, but as some article warns me, chkdsk deteled some of my unaccessible/unreadable files/folders instead of repaired or fixed it. so, i found extra space in my external harddisk for the files that has been deleted by the utility.

source:
http://en.wikipedia.org/wiki/CHKDSK
http://www.dataclinic.co.uk/hard-disk-failures.htm