External USB hard drive showing wrong "Free Space" "Used Space" inthe Capacity

The USB drive by Western Digital is the My Passport series, for Windows 7, NTFS formatted. It shows, and I've rechecked a half dozen times and looked for hidden files, that total capacity is 465 GB (taking the smaller number) and, for each folder (taking the largest number and even rounding up) the used space should be 160 GB, leaving about 305 GB. Yet Internet Explorer shows these two numbers reversed (almost, it shows 300 GB and 165 for used and free, respectively). Why? As best I can tell, somehow the partition tables or what not are screwed up. I'm pretty confident, having checked a dozen times, my eyes are not playing tricks on me.

I'm mildly curious to fix this, but if I reformat, I'll lose the excellent "encryption" tool that comes with every Western Digital external USB HD, that makes it impossible to access the drive unless you enter a password (I think the program is called "WD Unlocker").

What tools can I use to diagnose? Preferably free? I did check the online articles taking about "shadow" folders and what not, but they would only explain about 10% of the difference. This is a huge difference.

RL