MacBook Pro refuses to eject external disk - How do I make this stop?

So I have an issue that's been bugging me for a couple weeks, now (from even before I upgraded to Mountain Lion), that I, really, need a solution to. I have a 1TB external drive divided into 3 partitions. I eject it whenever I wish to remove it from my computer, but, lately, there is 1 partition that, constantly, refuses to be ejected. I get the usual warning in this instance:

The disk [volume name] wasn't ejected because one or more programs may be using it.

I click "Try Again" and it doesn't work. It works with "Force Eject", but I don't want to cause any unnecessary damage to the contents of the drive. The next time it happened, I ran the following in Terminal:

sudo lsof -xf +d "/Volumes/[volume name]"

This told me that the "mds" process was accessing the disk, which means Spotlight was using it. I clicked the "Spotlight" icon in the menu bar and Spotlight didn't appear to be doing anything. I can't quit the "mds" process in Activity Monitor, because the process just starts again. The only thing that worked was logging out, then, logging back in, then, I could, immediately eject the volume.

Of course, the next time I plugged the disk in, the same thing happened.

How do I stop this from happening? How can I stop Spotlight from, constantly, holding this disk captive? I shouldn't have to log out to eject a disk.

Please advise.