If you’re like me and have multiple copies of OSX on your machine (older versions, cloned backups etc), you’ll no doubt be aware that the Mac App Store detects all installed software on all partitions. This can prevent you from installing an application even if it’s not installed on the partition you booted from. Considering that the MAS installs apps to /Applications, why it searches your entire computer is beyond my understanding, but unfortunately there doesn’t seem to be a way to stop it from searching these partitions.
The easiest way to combat this is to eject the partition that contains the installed application (from Finder), or unmount it (from Disk Utility). Restart the Mac App Store, and it should allow you to install the software on the partition you booted from. Boom.
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