AFAIK all OSX apps had to be installed on the startup disk (with the exception of carbon apps.) But nowadays newer versions of OSX apps are starting to allow you to install elsewhere. I suspect also permissions or ownership of the drive makes a difference. I had problems running LW 4.10 from a partition which is owned by the system (I should change that whilst I think of it), whereas another partition I use for OSX apps is owned by me. I'm glad I own my own computer.