I've had really bad luck with the reliability of mobile HDDs in storage. Try to use them after 3-4 years and they just die. Maybe I've just been unlucky?
You do realize that HDDs can randomly break, right? To keep a proper backup you need to have multiple copies of all your data and also regularly access / copy it.
The only issue is that they have to be labeled clearly, or you are going to have a hard time to find whereabouts.