I have somewhere in the region of 500 albums in my iTunes library, the vast majority of which are rips from physical CDs. However I tend only to buy music from the iTunes Music Store nowadays and as such make sure I back those tracks up regularly.
iTunes itself offers this facility by an option on the Back up to disc command to back up only purchased music to optical media, however I back up to a NAS device so this doesn’t work for me.
So, the rather neat and geeky method I use is to drop down into Terminal and do the following:
iBook:~/Music/iTunes/iTunes Music aspender$ find . -name *.m4p | cpio -pdmv /Volumes/music/backup
This pipes the list of m4p files found in my iTunes library (.m4p being the file suffix used by the DRM protected AAC files you get from iTMS) to the cpio command which archives them whilst preserving the directory structure. The inclusion of the -p option performs a copy-pass function which un-archives the files into the target directory. Alternatively you could omit this and keep a single cpio archive file around.
Easy access to the powerful Unix underpinnings of OS X is one of the reasons I love Macs.
As for the rest of the library, well I’m planning a major exercise to re-rip all my CDs into a lossless format for future proofing, then do conversion into MP3 so I’m not bothered about backing them up for the time being. More on that later though.