J7n: no one can tell you what would be reliable enough. you could keep a perfect hard drive in a dry dark temperature controlled closet and power it up once a month and your house could catch on fire and destroy it. conversely, you could burn a set of dvds and toss them in a corner, dust them off 5 years later and wipe all the dust and crud off and they could still read...then you'd feel like the 2nd hard drive backup was a poor investment.

i think 2 copies of something is a reasonable backup. could something happen to 2 copies of the same data at the same time? sure...your house could catch on fire with your hard drive in it and your 1 year old could decide to teeth on your dvds on the same day...is it likely? no.

for my own personal use i think a hard drive and a dvd backup is secure enough. if I ever encounter an occasion where I lose both at the same time...well then that really sucks. but losing data under this circumstance is less likely than only having one copy.