My gut feel is that a large drive (anything over 50G to 100G) should be
implemented as a RAID array. 

I also like what PeeWee05 said about having at least 3 external copies: 
one at home, one at work; one with a trusted friend. 

I agree that a backup to an external HDD is far, far more convenient than
backup to CD, DVD, or evenTravan tape or other large capacity tape. 

In any case, a serious backup strategy should create a backup set on at
least 2 different external HDDs -- and 3 would be even better. 

--
Martin