How do I free up space on My Book WD 1TB drive?

I have an external hard drive (WD My Book 1TB) that should have about 931 GB of free space.  It is only showing 624 GB free even though I have nothing on it!  I deleted everything I had saved on there and emptied the recycle bin. When I look at it in Windows Explorer (I have Windows 7) there is nothing on it and yet right-click, properties is showing 307 GB used!  I need the space in order to do a backup.  How can I get rid of the so-called 300 plus GB of data on there?  Any help would be much appreciated, thanks.

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Reformat the drive 

How do you reformat the drive to start all over again. Mine is full. Have never emptied it in 4 years.