You can organize you music files your own way on your Mac and just have iTunes build the index so it looks organized in anyway you want iTunes, without duplicating your music files (MP3 etc) inside the iTunes database.
Just go to iTunes>Preferences>Advanced and uncheck the box "Copy files to...
I'm a long-time Windows user who just bought a Mac. Since I haven't figure out how to specify the download folder dynamically, I create a folder called DOWNLOAD as the target for all downloads. After multiple downloads, I would then double-click "Macintosh HD" icon twice to launch two Finder...
i-Tunes Advanced Preference has a check-box called "Copy files to i-Tunes music folder when adding to library" that is checked by default at installation. Un-checking this check-box will let you keep you music on a USB drive and only set-up pointers in i-Tunes. Must make sure you don't rename...