...to back it up you just go to export library?
I'm not sure what happens in that situation to any music you've purchased from the iTunes Store. Normally (I think) you have to "de-authorize" purchased music and then re-authorize it when you move them to another drive. Again, I'm not sure how that works because I never buy from iTunes Store. All of my stuff is ripped from my CDs, or public domain, or else free sample tunes.
It appears (I think) that the iTunes "Library" is an XML file that has a list of all the playlists, tunes, music folders, podcast subscriptions, radio station links etc. My XML file got corrupted once somehow and I lost all of the playlists and most of the podcast subscriptions. I ended up re-importing 1,800 or so files into the Library and had to go through and re-assign them to the playlists. Fortunately, many of them were able to be sorted by genre and/or other ID3 tags, which sped things up considerably.
I believe you
can "Export" the library file, from the "File" menu.
(running Mac OS X "Tiger" version 10.4.whatever's_current)