backup purchased music only
iTunes does not allow you to drag the entire contents of the "purchased" list under "store". At least mine won't. However, it will allow to "drag and drop" copy multiple songs from the main "music" list under "library".
A work-around is pretty easy:
1. Go to the "purchased" list
2. Highlight all the songs in the purchased list under "store"
3. Right-click "get info"
4. Under any category (I used "comments") type "purchased". You can use any other word/numbers/characters as long as they won't commonly show up in iTunes search results.
5. In iTunes go to "library">"music".
6. Type "purchased" in search bar and only your purchased music will be shown.
7. From here you can "drag and drop" copy all your purchased music to an external hard drive or any folder.
This is preferable to copying your entire iTunes catalog, especially when you have music that you imported from CD (no need to back that up).
The only disadvantage is that the copied files will ot be organized into folders by artist/album, etc...
IMO, it's pretty a ridiculous oversight/choice by Apple that iTunes only facilitates copying of purchased music to a CD/DVD...