You should keep a backup of your iTunes library on another drive (or two), because if your hard drive crashes, all your music is gone.
So start by figuring out the size of the backup drive you need. Check the size of your current hard drive (highlight the hard drive icon on your Desktop and select “get Info” from the menu bar) and see how much available room you have left, and then find out the size of your current iTunes library (by highlighting the Music folder in your home directory and getting info, like above). Add the two numbers together and get a USB drive of at least that size or larger.
Once you have a USB drive, you can do your backups by copying over the whole music folder. Plug the drive in and drag and drop the Music folder to the USB drive. That will make a copy of all your music which you can move back to your iMac after it’s repaired after a hard drive failure.
But I think you’ll find that dragging and dropping the Music folder to make a backup takes a lot of time. The reason is that simply copying the folder over like that copies everything, even the same music files that were copied last time.
For efficiency’s sake, when doing backups, any file that hasn’t changed doesn’t need to be copied over again. There’s no reason to make to much work out of making a backup.
So a better backup method uses a program to take care of the backup for you, and at the same time, a program that compares the dates on the files on your iMac and the dates of the files on the backup drive and only copies over the files that are new or have changed. That means if you only add one CD of music to your library in a week and then run the backup, the program will only copy over the one CD’s worth of music in a few minutes, and not the whole library, which would take hours.
If you use an automatic backup program like SmartBackup, you can run the backup anytime you want and only copy over the music you have added or changed since the last backup. Doubleclicking on SmartBackup launches it and stats the backup, and then SmartBackup can be set to quit when it’s done.
If you want to get very efficient, use SmartBackup in conjunction with iCal (the built in Calendar program on your Mac) so you can schedule a backup everyday. Leave the USB drive plugged in all the time and set iCal to run the backup during the day or night when your iMac is on but might not be too busy.
And if you want to be really safe, keep two backups, and keep one of them out of the house, worst case scenario. Your backups are really only as good as they are safe and out of the danger of being lost or damaged.