Many Mac programs only seem to allow themselves to be installed to the system drive (whenever you get that "Checking hard drives..." and then it seems to inevitably only show the system drive as an acceptable install location).
OTOH, it seems like one can generally drag an installed application to another drive and it launches fine. What's going on? If one installs a large program to the system drive and then drags it to another drive (triggering a copy) and then deletes the copy on the system drive, is that all it takes? Or are there some files still on the system drive, some on the other drive, etc.?