Aannnd, have more than one backup. At least one backup drive that I had —plugged into the computer— died at the same time as the main HD.External drives die, just like all drives die. Every read and write cycle means a tick of the timer. Each time it is powered on, it is yet another tick of the timer. It can, and will, eventually fail.
Being stored externally is nice and all, but it's inadequate if you want to prevent data loss. That's just the first of three fairly important steps. Also, it's important to test backups. An untested backup is not a backup. You need to be reasonably certain that you can restore your data from said backups.
Bad power supply fried them both simultaneously. And also, at the same time.
Lots of folks have their lost data stories.

