Software developer, musician and photographer based in Boston, Massachusetts.
Here’s the (relatively) short story for all you people using The Google:
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.
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.