The Mac is supposed to be all about simplicity and ease of use and a nice useful graphical interface. And while Garageband almost fully accomplishes this goal, there are things that we still have to do the "unix" way.
Rebuilding an index should be accomplished through a simple button marked "Rebuild index", perhaps located somewhere in the Advanced Preferences so it's not part of the music making interface of Garageband. That would make WAY more sense, than telling people to go to all sorts of folders they normally never touch and to drag files to the loop browser, etc.
Apple didn't even bother to add the same technology they have in iPhoto, for detecting duplicates, so you can end up with triplicates of loops like I now have. And there is no "easy" way to remove them. I have to use a "unix" approach of creating a diff between two directory listings. I can't find a "Find duplicates" Finder option anywhere either! But that's a bit off topic Just another example of how Apple needlessly 5% of our computer usage.