Soooo to those of the technical persuasion, what is the best way to prolong the life of my backup external hard drives? Should I only leave them on when I am actively using them transferring data, leave them on all the time, something inbetween? Yes I backup everything to DVD's as well, so I'm not solely relying on these for my data's safety, but I'm not exactly rich enough to afford new 500gb hard drives on a whim.
This year you aren't rich enough, next year they'll be under $100.
I've never seen data on whether drives prefer to run all the time or be power-cycled on for occasional use. The manufacturers claim to rate them with a Mean Time Between Failure number, and it's in hours; so you'd figure, if you're going to use them for an hour, and then not use them for 23, best to shut off. But the manufacturers do capture the number of times a drive is powered on in the S.M.A.R.T. data kept on all modern drives, so they consider that meaningful.
All that said - in an external enclosure heat is more likely to be a problem. So my vote is, power it off.