It's often better to buy a good internal SATA drive and place it in a good external USB enclosure than to buy a ready-made external drive (not only quality-wise, but sometimes even in terms of price as well). Also, keep in mind that the hard drive failure rates that ssj posted are all for internal SATA drives.
It's also a good idea to back up everything.
Currently, I have two internal SATA drives inside my computer itself. One is the main C drive (a 1 TB WD Black), and another is a 2 TB Seagate Barracuda that I designated drive F. I use drive F to store my media files, documents, and virtually everything else important.
I have a 1 TB WD SATA drive designed drive R in an external USB 3.0 enclosure, which I use to store system image backups of my C drive that I make using the free version of Macrium Reflect.
I also have an external USB 3.0 drive containing a 2 TB Seagate Barracuda (designated drive Z), which I use to back up everything on drive F (using FreeFileSync).
This way, if my system drive fails, I have system image backups of it on drive R, which I can use to turn any replacement drive into a clone of my old one, operating system and all. If drive F fails, I have everything on it backed up on drive Z (or at least everything from before I last ran FreeFileSync).