I too use a single internal hard drive and use
Genie Timeline to continually feed changes to an external drive, performance impact is negligible on my PC.
To be honest though (famous last words...) I've never had a hard drive fail on me. My internal 1TB Seagate is 12 months old, 1 of the external drives is 250GB Maxtor, nearly 7 years old, the other 2 are 250GB Maxtors nearly 4 years old.
Tony