It's important to think about adverse events other than drive failure.
If you are burgled, it's likely your PC and your external hard drive will be stolen. If you have a fire or a flood then both devices will be damaged. Insurance will replace your PC and hard drive, but can't replace your data.
So make sure that some back-up is off-site - for example Dropbox, Skydrive, BT Digital Vault, Flickr, whatever, or burned on to DVD and left at another address, or...... As stated earlier in this thread, don't rely 100% on the Cloud either as the company may fail and then your data evaporates!