Then what I would do is to buy another HDD exactly the same size like the one youre using and then use your motherboard to run them in RAID 1.
Then youll have two identical cloned discs meaning that if one fails, all the data will still be left on the other drive and all you need to do is to run it as a regular single HDD to access the data again. It also means you will increase the read spead of you drives as the system reads from both simulatenously.
With an external USB hardrive, youll just neglect to do regular backups because itll be so slow to transfer data, and when the main HDD breaks youre left with your pants down.