And yes, adding an external drive will spread the risk, however bear in mind that it too can fail and that you should back up critical files on something more permanent like CD or DVD.
I had an external hard drive fail. I'm not sure that it was even out of warranty. But if a failed drive contains (ahem) "sensitive information" that you don't want other people to see, you may not want to entrust your drive to someone else. That was the case with me, so I just took the drive apart and messed it up so that nobody who found it could get anything out of it and threw it away and bought another.
If you have valuable files that you are storing exclusively on the external drive and not also on your computer, I would recommend getting a second external drive to back up the first one.
Edited to add: These were drives of 100 to 200 GB, not these terabyte things.