Mac App Store confuse with partitions/external disks

I have a MacMini with 2 HD.

One with El Capitan and one with Mavericks.

When i installed El Capitan i realized Mac App Store by default update all the applications even those in Mavericks Partition.

So i unmounted the Mavericks Partition (and changed the preferences) and

I re-download an application from the Mac App Store from my ElCapitan Boot.

Once downloaded it was not on the folder Applications.

But in a folder (Volumes) with the name of an external disk. (not plugged during the installation) I found it, by launching the app via LaunchPad and Show in the Finder via the Menu in the Dock.

This Application was on this backup external disk, it's true. and the disk has be plugged at a moment or another. But it was not plugged during this installation.

I can see this fake Volume in Terminal but not with DiskUtility.

If i Plug the disk, i have in /Volumes : ExternalDiskName and ExternalDiskName2 !!!

How can i suppress this fake (ghost) Volume !

And why Mac App Store is looking everywhere and not only on the system partition. It's a mess !

Thanks !