All right, so my question is half-answered here: https://discussions.apple.com/message/23588511#23588511
However, I already downloaded the "orchestral" set that seems tied to the Jam Packs. Will downloading it again via the Jam Pack downloads override it, or will I have 2 versions of the same set? And besides restarting my computer, is there anything else I should know in order to have everything set up properly in GarageBand?
I'm going to bump this one more time. I'd like to make sure that I'm not putting my computer in any danger before I attempt anything that I shouldn't attempt.
Before you do anything with the library files, be sure your backup is current and includes the libraries, not only your documents.
It is hard to answer that question, since most of us cannot experiment with your setup - the latest GarageBand versions are GB '11 and just recently GB 10.0. I don't remember, if the Instrument Library was in a different place in GB '09.
And most of us have downloaded the JamPacks already as stand-alone boxed versions or as part of MainStage 2 . I don't know. if anything changed with the upgrade to MainStage 3.
For MainStage 2 it was sufficient to reindex the loop library. The MainStage 2 installer installed the instrument library, where it was needed for GarageBand '11, so nothing needed to be done.
In GB '11 the Instrument library will look like this, compare to your version - try to match the files you are seeing in the Logic folder to the files you are seeing in GB '09 and copy - don't move them - to the corresponding folders in Application Support > GarageBand.
The Apple Loops, that come with the JamPacks, should be installed in /Library/AudioApple Loops/
(see this GarageBand, Logic: Where loops get installed)
Check, if you are seeing additional Loops in other Folders and rebuild the Loop Index, see Rebuilding the Loop Index in GarageBand
It seems that GB '09 does follow the same path as GB'11, so in theory that should work! Thanks for the heads-up on the copy-not-move deal; I was fully prepared to delete the Logic versions to save space. Though it seems that MainStage gives me the option to download the instruments and not the loops if I wanted to, which I would like to do in order to save space (as you can guess, I'm dealing with a 2009 MacBook Pro with half my memory used up). Is it crucial to download the loops as well for it to work?