You can move your music library to another hard drive, but realize that you’ll need that hard drive plugged in whenever you want to listen to music. This also goes for podcasts and movies in iTunes; if you move the iTunes Music library, the hard drive must be plugged in to use any part of iTunes. (If you don’t, iTunes will prompt you to either locate the old library or to create a new library, which will result in an empty library.)
But if you’re outgrowing your hard drive because of your music, moving your iTunes Music library to another disk will give you much more room without having to replace your internal hard drive.
First, get a USB drive that’s the size you need. Drives get cheaper every few months, so get one that is big enough to begin with so you don’t have to move your library again anytime soon. You can check the size of your existing iTunes Library by going to your hard drive, and then to your Users, and then to yourusername/Music/. Select the iTunes folder and then “Get Info” from the Finder menu under “File.” Find the size in gigabytes (GB) of your existing library and figure out how much more room you need to grow, and get a USB of that size.
But before you move your iTunes library to the new disk, there is some housekeeping to do. The first step is to make sure all of your music and media is located in your iTunes library. If you have sometimes added music or media to iTunes and not copied the media into iTunes, that media is still located on other parts of your drive.
From the iTunes menu, choose Preferences. In the resulting window, click the Advanced button. Select the “Keep iTunes Music folder organized” checkbox. Click OK, if it isn’t already checked OK. And check – or be sure it’s checked – the box that says “Copy Files to iTunes Music Folder when adding to Library.”
Then click OK. Each option may result in iTunes doing some work in copying files, so give it a chance to work.
Now, to change your iTunes Music Library to the external disk, plug the new disk in. Remember that you’re going to have to keep this new drive plugged in all the time to use iTunes from this point on.
You now can use iTunes to copy your Music library folder to the new drive. If you have lots of media, this can take a while. The instructions below are from Apple KB article: iTunes for Mac: Moving your iTunes Music folder and are for iTunes 8. When it’s time to navigate to the new location of your music folder, go to the new USB drive:
1. Open iTunes.
2. From the iTunes menu, choose Preferences.
3. Click the Advanced button in the Preferences window.
4. Click the Change button in the iTunes Music folder location pane.
5. In the Change Music Folder Location window that appears, navigate to the location where you would like your new Music folder to be created. Note: By default, your iTunes Music folder is a folder named “iTunes Music” in ~/Music/iTunes/ where the tilde “~” represents your home directory.
6. Click the New Folder button in the Change Music Folder Location window.
7. In the New Folder window that appears, enter the name of the new iTunes Music folder.
8. Click Create.
9. Click Open in the Change Music Folder Location window.
10. Click OK in the Advanced window.
11. From the File menu, choose Library and then Consolidate Library… A message appears that says: “Consolidating your library will copy all of your music into the iTunes Music folder. This cannot be undone.”
12. Click Consolidate. Important: This action copies all of your music and media files to the new location. There must be enough hard disk space available to copy all of your music and media files.
13. After the folder has been copied, locate your original iTunes Music folder, and drag it to the Trash or Recycle Bin. Note: Don’t remove the iTunes library files that may be in the same location as the iTunes Music folder. For more information about the iTunes library files, see this article.
14. Empty the Trash.
To be safe, don’t delete your old iTunes Music library for a while, until you’re sure that all files got moved OK. If you manually placed music and movies in your iTunes folder without dragging them into the iTunes window and importing them, they probably didn’t get moved, because iTunes didn’t know they were there.
For iTunes 9, consolidate your Music library first. (See Apple’s KB article iTunes Mac 9.0 Help: Consolidating your library). In iTunes, Choose File > Library > Organize Library and then select “Consolidate files” and click OK.
While you’re at it, you can also choose to “Upgrade to iTunes Media Organization,” which means iTunes will organize your iTunes Music folder into the new structure of subfolders for Audiobooks, iPod Games, Movies, TV Shows, Music, Ringtones, etc.
Then you can change the location of your Music Folder to the external drive with the instructions above.
There’s an extensive rundown on moving iTunes folders, upgrading to iTunes 9, descriptions of how iTunes organizes music and track data and more by Jesse David Hollington at Transferring your iTunes Library | iLounge Article
You should keep a backup of the external drive, because that’s where all your music is now. So I’d get another USB drive of the same capacity and either do manual backups by copying over the iTunes folder, or better, use an automatic backup like SmartBackup that you can run anytime you want and which also will only copy over the music you have added or changed since the last backup.