Many people use a single external drive as a backup. The drive sits next to the PC it is backing up. A fire/flood/whatever can take them both out.

If you're going to use a HDD as a backup make sure you have multilpe copies (say three) with at least two being offsite. That way if your home/worksite is destroyed your data is on two other HDDs away from the calamity. It is unlikely that two HDDs will fail at the same time, but just having one HDD for irreplaceable data is just a big risk.