Windows 8 Backup

I just purchased a PC/Tablet with Windows 8.  I also bought a WD Passport external drive with 1T of memory.  When I try to create a backup on the Passport, I get a failure message that says there is not enough space?  There is 931 GB free on the harddrive.  Any suggestions about why this is not working?  Thanks!

How are you backing up with Windows Backup or with software?