Interesting barney, I have those same feelings. You know, now that I think about it, before I noticed any problems, I swear I had about 200GB of space left. Now, with each test I run, it seems to be shrinking, or at least changing.
4wd & Stoic: I do, thankfully, have the exact 3TB spare sitting here in its package. I was saving it for a charity education project I wanted to work on, but I'll use it now for this problem.