As an aside, and to restate a general principle: having a single copy of a file, regardless of the location of said single copy, means you have no backup.
One should not, if something is truly valuable, keep only a single copy of it whether on an internal disc drive, thumb/jump drives, an SD card (of any specific SD format), etc.
From the sounds of the original poster's last message he or she has files on their external drive that are not on the internal hard drive of their computer (or the internal storage of some other device) as well. That is just asking for trouble if you have a failure of your backup drive itself.
For "the world would end if I lost access to this data" type information a minimum of three copies should exist: the original, one on-site backup, and one off-site backup as the minimally safe "cover all your bases" setup.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.
~ Mark Twain