I have installed Mac OS X an additional HDD. So my MacBook now has two copies of OS X, and I can select which one to boot when it starts – great.
The only catch is that, if all my hard drives are mounted, the App Store app doesn't work very well on the newer install – it thinks apps are installed when they are not (they are however installed on my old install). Obviously it is seeing the applications on the second HDD.
I don't want to unmount the second HDD (which fixes this problem, but forces me to stop what I'm working on). I've tried adding the second HDD's Application directory to my spotlight exclusions to no avail. What other ways can I solve this?