F: is empty 97GB but Norton Ghost shows it has less than 10% free space

I set my backup destination on F: which has 97GB free space. However Ghost tells me that 'The backup destination on F: has less than 10% free space.' Please help me with my problem. Another problem of Ghost has been posted also by me. http://community.norton.com/norton/board/message?board.id=other&thread.id=7617

 

Winston