When I got the computer, I bought 2 160gb SATA drives. For a few months, I had the one hard drive shared on the network and the other one was just for backup of that hard drive. Now, I have one hard drive in an external enclosure and I use it 3 times a week for 20 minutes each time. Should I switch the hard drives and use the backup hard drive for like a month or two? (The computer is on 24/7) and let the other hard drive act as a backup drive?

What kind of backup are you doing? I had my computers set up with a two partition primary HDD. Periodically I would back up the primary partition of this drive containing the OS to the second partition using Ghost 2003. You could also cook these images to DVD or write them to an external USB 2.0 HDD but that is very time consuming. I now have a second HDD in my computers that has its own power switch. Periodically I will clone my Primary HDD to this backup drive. The assumption that I'm making is that if the primary drive does fail, there will be a good backup clone of my drive on a HDD that hasn't been used very much.

I have a 20gig hard drive running windows. 1 SATA drive is in the computer and the other SATA drive is for backing up the one. But I was thinking along the same lines, the second one should last a lot longer.