Windows 7 - External HDD Shows incorrect amount of free space?

I just recently created a Windows System Image backup and stored it on my External HDD. However, after this occurred, it seems as though Windows has reported an incorrect amount of free space left. My drive is has 931 GB of total space (1 TB in base 10) and currently reports 272 GB of free space. However, when I select all of my files and folders in the volume and select "Properties", both SIZE and SIZE ON DISK report 639 GB used, which would mean that there is supposed to be 292 GB of free space. There are no hidden folders, and in addition, chkdsk /f didn't seem to find any problems. I heard that formatting generally fixes this but it is not a feasible option at all. What are my options?

That 639 gigs used might be the problem instead, 639 gigs used not counting all the extra space taking up by filenames and such.

Here's a link to download a free partition manager, one I use:

Download Active@ Partition Manager 1.3.12 Free - Disk management software. Create, format, and delete disk partitions. - Softpedia

See what free space shows up using that, I'll believe it's reading which will appear in the top section.

This is normal. Most of the calculations are estimates anyway. Plus you have to remember the MTF is being updated as well.