Hi dido mamoun,
Are you using WD Smartware to password protect your external drive?
Have you tried to remove the password protection with WD Smartware and checked whether it helps to access the drive? Setting up security to lock or unlock a WD external drive with WD SmartWare
Smartware, as I understand, has always been a problem and pest many people loathe.
In any case, it does not hurt to run Data Lifeguard Diagnostic for DOS
and check the health of your drive as a first step as Britton30 wisely counselled.
Download from http://support.wdc.com/product/download.asp?groupid=604&sid=3&lang=en
Create a bootable MS-DOS pendrive with Rufus Rufus - Create bootable USB drives the easy way
, unzip the download into the bootable pendrive, boot from it and then against the command prompt type dlgdiag5.exe and press enter to run it. ( Do only the Quick and Extended Tests. Do not write zeroes which will destroy all data.)