I want to delete everything on my book live so I can try to back up all my iTunes music and movies back to my book and use it as my external hard drive so i can free up memory on my iMac
You can delete all content at once by restoring your WD My Book Live to factory defaults in the unit’s Dashboard (Settings> Utilities> Factory Reset).
However, bear in mind copying files to your NAS and deleting them from your system to free some space means you would not have a backup of your data.
I backed up all my iTunes music and films but when I tried to play them on my computer or put them on to my iPod I couldn’t find them hence why I want to delete everything and start again
ITunes music and videos is stored by default in a folder called iTunes Media on the computer where the iTunes program is. You need to google and find out more about this. The iTunes Media folder on my NAS is a COPY of that folder; the original is untouched and still used to catalog my media files. Periodically, I refresh that copy to keep it up to date.