How do you back up your photo files?

[Green_Elite_Cab]

On multiple external hard drives. A new primary drive is purchased once a year, and the one-year-old drive is demoted to secondary status. A two-year-old drive is used for off-site backup, which I keep in my office. Upon reaching three years of age, a drive is retired.

Do the drives go bad, or is this just a failsafe? I assume you resell the old drives?