Mac App Store installed app on wrong volume

I recently updated ReadKit through the Mac App Store (MAS) and noticed immediately that it was no longer actionable via LaunchBar (gasp)–LaunchBar just wasn’t seeing it anymore. The MAS said it was installed, but I couldn’t find the application package in /Applications or ~/Applications.

For whatever reason I thought to use Apple’s Launchpad (something I’d never used before) just to see if maybe it was aware of the application; it was, and launched it just fine. At this point I was seriously confused: ReadKit was installed somewhere (accessible to my Mac), and was being indexed by Launchpad, but not LaunchBar. Hrm.

I next thought to jump into OS X’s System Information app and look at the list of installed applications. (Click the apple icon in your menubar → About This Mac → More Info… → System Report… Once you have the System Information app open, look for “Applications” under “Software”.) ReadKit was in the list, but—and here’s the kicker—it was installed on one of my backup volumes, and nowhere else. Every other app appeared three times in this list: once on my Mac, and once on each of my two backup volumes.

I have no idea how this could have happened, but it was what it was. Now, how to fix it? There probably are a few ways to resolve this, but the easiest—I assumed—was to delete the copy on my backup volume, unmount the backup volume, launch MAS (which no longer saw the app as installed), and reinstall like normal. Worked like a charm.

If you’ve any theories as to why this happened in the first place, I’d love to hear them.