How to install app from App Store again on Mountain Lion (or on the new OS X)? I had the game "Plants vs Zombies" purchased on the Apple App Store (for Mac) when the computer was using Lion.  Now I made a new partition and installed Mountain Lion on the new partition.

And when I use the App Store to try and install the game on Mountain Lion, it says it is "Installed", and using the "search" at the top right part of the screen, it actually can find "Plants vs Zombies" and even won't list that it is on "Mac HD" (some apps will show "Mac HD" to tell it is not on this new partition of Mountain Lion, which is call "Mac HD 2".)  So it can let me run it... but I think it may be less clean that way.

So is there a way to install it on Mountain Lion?  I think it is more robust if the app is installed on Mountain Lion instead of relying on an old partition.  (what if the old partition is to be deleted a year from now?).  If I Optionclick the "Installed" word on the App Store, it will not do anything.

(I also don't want to just "copy" the app from the old partition to the new partition, as it is some what a brute-force copy and may not be too robust that way).
I've run into this before, and there really isn't a perfectly clean way to do it. However - if I wanted to trick it...
If you unmounted the disk/partition that had the existing copy on it, then App Store couldn't find it. Have you tried renaming the existing copy, or temporarily moving it off the Lion partition to USB key, then rerunning App Store?