Disk Uses More Space Than Size of Files

I have a 1 TB NTFS partition on Windows XP that reports through Explorer
Properties dialog as having 70GB available. When I add up the size of all
of the files on disk, there should be 350GB available.

I am very aware of cluster sizes and how many small files would take up the
minimum cluster size, usually 4096 bytes per file. The problem is the
partition in question only holds huge backup files, minimum 1 GB in size.
So there are no small files on the partition that would waste empty sections
of each cluster.

I emptied the Recycle Bin, so deleted files are not accounting for this
issue.

What would account for the waste of space being reported by the OS? What
tools might help me to explore this further?

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W