Honestly, at this point, if I were you, I'd be pricing replacement drives because that one sounds like it's in the very early stages of failure. Five or six years out of an external HDD is an astonishingly good run however. Usually externals don't make it more than 2 years.

You could try, since you have backups of everything on the drive, wiping it clean and recreating the partition structure, then restoring the files from your backup, but I doubt that will solve the problem. It will probably just move the problem to a different partition. Still, you have backups, there's at least a chance it will resolve the problem, and it is a free solution. If that doesn't do it, then you can start looking for a new external.