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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.?

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up vote 2 down vote accepted

Well-behaved applications don't care where they are installed.

However, some application installers install other components in the /Library folders for whatever reason (dynamic loading, external tools or extensions, special configuration, etc.) and thus require themselves to be installed on the system drive.

When you copy the "application" portion of these files to another volume, these other installed files could still be found because of whatever preference configuration the application uses.

This is why I prefer that Applications install such external tools on launch rather than installation. (E.g., BBEdit will offer to install/update command line tools.) The best installers are simply drag-and-drop.

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Sadly, there are a lot of poorly behaved applications, including many of Apple's. I should be able to make subdirectories on /Applications to try to organize the mess, for example, but most updates will either ignore the subdirectory (Apple does this a lot, with odd results due to multiple programs with the same bundle ID) or occasionally move it back into the sump. – geekosaur Mar 31 '11 at 20:11

When installing a program like that, hit Command-I (or Command-L, I don't remember offhand, the menu option should be in the File menu) to see the list of all files that will be installed and where they will be installed to.

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