Here’s the (relatively) short story for all you people using The Google:
- Find the iTunes Music folder location in iTunes Preferences under the Advanced:General tab.
- Click Change… then navigate to the location on the external hard drive where you’d like to store your music. Create a new folder.
- Ensure that the Copy files to iTunes Music folder when adding to library option is checked and click OK.
- Go to the Advanced menu in iTunes and choose Consolidate Library….
- When prompted with “Consolidating your library will copy all of your music into the iTunes Music Folder.” click Consolidate.
- Once the copying finishes take a look at the folder on the external hard drive where you put the music and do a spot check to ensure all your music was copied over.
- On your notebook’s hard drive, open Finder and go to your Music::iTunes::iTunes Music folder and free up hard drive space by deleting as much or as little of your music as you like. Don’t delete any music that you’d like to be able to take with you when the external hard drive isn’t attached.
- There is no step 8. When your external hard drive is attached you can play any song (or Movie, or TV Show, or Podcast, etc.) from your music library. When you’re external hard drive is not attached you’ll be able to play any song (or Movie, etc.) that you kept in your Music::iTunes::iTunes Music folder.
If later you add new songs by ripping from a CD, buying from the iTunes store or whatever when your external hard drive is not attached, you can add them to the external hard drive simply by re-consolidating your library when your external hard drive is attached. Just go to the Advanced menu and choose Consolidate Library… again.
Note that if you previously changed your iTunes Music folder location to something other than Music::iTunes::iTunes Music, you’ll need to move your music back to Music::iTunes::iTunes Music between steps 5 and 6, otherwise iTunes won’t be able to find the music you kept for when your disconnected from the external hard drive.
The Back Story
Back in 2006 I responded on Ask Metafilter to a question someone posted about keeping a larger-than-would-fit-on-the-PowerBook-hard-drive music library in sync between a PowerBook and an external hard drive. The questioner wanted to keep a subset of the music on the PowerBook for use when disconnected from the external drive and, when connected, wanted to be able to access the entire music library through iTunes.
Having helped a friend with exactly the same problem I had the solution, but in my answer I left off the crucial bit about using iTunes' Consolidate Library… feature to do the copying. Instead I had written that you could just copy the music to the external hard drive yourself. As I discovered yesterday when trying to recreate this setup on my (now full) MacBook, if you copy the files yourself the iTunes Library doesn’t get updated with the new file locations. Then when the external drive is disconnected iTunes can’t find your music. Updating the iTunes Music Library XML file in the Music::iTunes folder to reflect the new location doesn’t help either. It seems the only way for iTunes to update its library is for it to do the copying itself.