which is the correct one?
They are:
[1] MacintoshHD/Library/Application Support/GarageBand/Instrument Library
[2] MacintoshHD/System/Library/Application Support/GarageBand/Instrument Library
[3] /Users/swgutto/Library/Application Support/GarageBand/Instrument Library
Instruments should be installed either in [1] or [3].
[3] is the user library. If you install instruments there, they will only be accessible by you.
[1] is the system-wide installation library and should be used, if you want to install for all users.
[2] is a private system library, and software extensions should not be installed there. On my Mac is no "Application Support" folder in the private system library.
The "Thumbstroke Bass" is probably one of the JamPack Rhythm Section instruments. I cannot tell from my current installation, where GarageBand '09 installed this instrument. Search with Find Any File for this audio file: DPTUXLO1A2X02.aif . On my mac I found it at
/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Files/Thumbstroke Bass/DPTUXLO1A2X02.aif
But that cannot be the original installation place.
You can download "Find Any File" at -http://apps.tempel.org/FindAnyFile/index.php