New 2TB Hd now showing 1TB

Hello all Any ideas to help would be great, I brought a 2TB caviar green HD three weeks ago as a data backup drive and it has been working fine until today. When I turned on the computer windows 7 Gave me a popup saying that the a new drive needed to be formatted, Check disk management and it has stated the drive is now Only 931Gb. in size. Check BIOS setting and it is also saying the drive is 1000Gb ??? Changed the SATA  ports the drive was using and this made no difference. Also ran the WD Diagnostic and it telling me he HD is fine. any ideas or just do a warranty clam.

See Q2:

thank-you fzabkar it worked a dream. Something else of intrest after doing the fix on the next reboot I kept geting MBR error 1 comming up, it seem to want to boot form the backup drive even though I was telling the bios to boot of the correct drive, Did a bios hard reset and all was fine after this, once again thanks