hi, i needed to do this recently, I wouldn't want to tell you from what I remember because I'd probably mess your computer up, but I'm sure I got all the information from http://www.apple.com/support
If you're using a Mac try searching for a freeware/shareware program called "Senuti" (iTunes backwards). If on Windows, try going via the senuti website, I think there may be a link there to something useful.
Hope this helps
fo3nix, if you do that then all the files will be corrupt and you'll have to remove them all from itunes and start again. however it might be possible, I'm sure you need to go into options in itunes and change the path to where your music is stored.
He's just wanting to make a backup, and that'll all it'll do. It won't alter iTunes in any way. If you need to import the music again, just copy across by dragging the music on the external drive into iTunes. (This is assuming that iTunes is set as default to copy new music into iTunes Library location.)