I know I'll get flamed for this but using a single portable hard drive is not the best way to backup anything.  The best way is to create multiple copies and store them at different offsite locations.  The best and easiest way to do that in today's world is to use an online backup service.  There are numerous ones available but I use Google Drive for 95% of my important files.  The files are replicated on each of the devices I own plus stored on a cloud server.