If memory serves. iTunes itself can rearrange things into artist and album folders -- but you have to allow iTunes to copy things into, and then manage, your iTunes Music folder.
Failing that, you can use a tagging application (such as mp3tag, which is available for Macs running Wine -- but that doesn't work for Catalina, and might not work on a G5) that is capable of using the embedded tag info to rename things into folders appropriately.
On the Mac, if you open a Finder window, and navigate (under your home directory) to Music, iTunes, iTunes Music -- working from memory here, please allow me some slack :^) -- somewhere down there should be the folder structure that the tracks are actually stored in. Does that look like artist -- > album --> tracks? If it does, you could just open another Finder window to your USB drive and drag (copy) everything across exactly as is. If it looks like somethig else, let us know.