Unless you are using your system for some sort of business, it is doubtful that you would ever need more than a fraction of your HD space. If you have a bunch of photo's, or video's on your system, that may be why you are using so much HD space. I take a fair amount of family pictures, and videos, but once I view them on the computer, I move them to a CD or DVD, and erase them from the HD. Pictures and videos gobble up a lot of space on the HD. Clean all the old unused stuff off your HD, then erase the External, and then, do a backup...you will probably find plenty of room left over.
Once you have everything cleaned up, the first backup will copy everything on your HD. After that, the backup will just update/modify files you have added or changed. As I recall, the first backup I did with W10 took about 15 minutes, but now when I do a backup, it is usually done in less than a minute, and barely adds anything to the external drive capacity. If/when I start to approach the External HD capacity, I will just open the Restore option, and delete backups dating back weeks or months.