New WD 5TB My Passport can't backup

Just purchased a new WD 5TB external hard drive and plugged into USB on a Windows 7 PC. It shows correctly with all space free to use but it will not allow use as a backup. It seems the PC does not recognise it for this purpose. Can anyone suggest what has gone wrong?

Hello,

Please refer to the following KBA article: http://support-en.wd.com/app/answers/detail/a_id/3653/