You might be able to move non operating system files over onto a standalone hard drive, be it portable or powered.
By non OS I mean any multimedia, Photo’s, Video’s etc.
You could try right clicking on C drive, Properties and, in the middle of what you see, there should be a “Disk Cleanup” button.
If you click this, it will open another small dialogue box and show you what old installation files there might be, as well as Temp internet files, Direct shared cache and other items that might free up some Hard Drive memory.
Your only other option would be to clone your current hard drive and upgrade your current hard drive to something larger. (Pretty straight forward on a desktop. A bit more dismantling involved if a laptop).
Also, if going down the external hard drive route, don’t let these become 100% full, as I’ve had them lock up on me in the past. So I only fill to about 90% now.