Then I looked at (but didn't try) the script from the previous posting Move iTunes audio files between hard drives via script. None of the solutions seemed to do exactly what I wanted to do.
I then realized that iTunes would do it for me, with very little trouble. First I created a playlist with all the content I wanted to move, then exported that playlist to an XML file (File > Export). I created a new user account, logged in to it, and started iTunes. I set the preferences so the new iTunes would copy all imported tracks to its local library (and made sure it was located where there was plenty of room!) and imported the XML file created in the first step. When this import was finished, all of the files and associated metadata I wanted to separate were copied to a new location.
I went back to my normal account and deleted the content of the playlist by selecting all the tracks, then pressing Option-Delete, which asked me if I wanted to actually delete the files from the library and put them in the trash.
Simple, no script, no muss, no fuss. My library is split, and it's now a little more manageable.
[kirkmc adds: I'm in a similar situation, but I actually am considering consolidating my libraries, and this hint will allow that too, as well as simply transferring some music from one library to another. I have about 200 GB of music, half classical, half "other", split into two libraries.
A few comments about this hint:
First, you need to make sure that the new account can read from the location where the original music files are stored. In my case, I use a separate hard disk for my music; this is probably what the poster does, because he didn't mention any issues. If, however, the music is in your Music folder, another user account won't be able to access it because it won't have read permission. Which leads to...
Second, while I used two separate accounts for a long time, I later switched to separate libraries managed from the same account (if you don't have a second hard disk, using the same account will prevent any of the above permissions problems). There are two ways to do this: either use iTunes built-in multiple library feature, or use something like Doug Adams' iTunes Library Manager.
Finally, it's worth noting that this method stores all the metadata associated with the files; not just the actual track info, but also your rating, play count, last played date and more. So if you want to either split or consolidate your libraries, or if you simply want to move music from one library to another, this is probably the best method for doing so.]

