A good backup system ALWAYS should make provisions to isolate the device carrying the data for safety, protecting it from electrical glitches, etc and physical damage. Among other things, that means you ought to be able to disconnect it completely from any electrical supply and from the computer. Ideally, the unit should be physically stored in a different location to guard against rare events like fire, flood, etc.
For these reasons, an external HDD makes much more sense, even if you don't store off-site.
You want an example of rare dangers? I heard a story that one professional data center operating in one of the World Trade Center towers did it all right, including storing backup tapes off site - in the other tower!