You know your falling into a false sense of security with the external hard drive right?
An external hard drive has the same faults and issues an internal hard drive has. Ya, the chances of both going at the same time are relatively slim, but you aren't really all that safe by just having things you will miss on a backup hard drive.
If its really important I would burn them on DVD's or something and keep the data somewhere safe. And if your really paranoid then make two copies and keep them in two physically different locations i.e one at home, one at work or safe deposit box or something.
Having a backup is good, but your falling into a trap if you think that a hard drive is the way to go for extremely important data.
Not that dvd's or cd's are perfect, but they have a relatively long shelf life and they are cheap to buy with relatively large storage capacities.