Norton Ghost 9.0
I have 4 x 200gb internal hard drives and an external 200 gb firewire.
internal 1) Win XP DAW
internal 2) DAW Data + Audio Files
internal 3) Win XP DAW (clone, offline,)
internal 4) DAW Data + Audio Files (clone, online, scheduled for daily backups)
external 5) DAW Data + Audio Files (manual backups weekly, stored offsite)
This way even if my XP drive fails in a session, I just plug in the power cable for the clone drive and away we go.
People backup their audio and data, but backing up the actual DAW, settings, plug ins, etc., can save you a whole lot of heartache.
I used to use a RAID array but Ghost is just more convenient in my experience.