But, Noozer + Kony:
Hard drives fail. DVDs and CDs have no moving parts and don't fail. Backups
need to be 100% reliable and a hard disk isn't.
DVDs and CDs have at least four common failure mechanisms.
1) Bending produces hairline cracks and they may shatter in
a drive once spun up.
2) Scratches to the top may destroy the data layer.
3) Instable dye may turn bad over time.
4) Instable coating may delaminate.
I've had plenty of discs just go bad sitting in a humidity
and temp controlled room, not exposed to strong light,
almost no sunlight at all in the room... sitting in their
case in a CD rack. They read fine right after being written
and that's the problem, that they seem ok until they've aged
awhile... generally it took around a year or longer.
IMO, the best backup is to use a RAID1 or 5 array of hard
drives, like a NAS or fileserver, AND a separate offline,
not-regularly-powered hard drive(s).
Perhaps once HD-DVD burners and media are inexpensive, it
will be less of a bother to make redundant copies of discs.
As it stands today, doing that with DVD+- just takes far too
many discs for many purposes, and a lot of time. I find the
more time a backup takes, the less often I do it in a timely
manner instead of putting it off.