Hi
I'm a big fan of free software, but backing up my computer is so important that I purchased Acronis True Image.
It's very easy to use and I think they have a trial version so you could make sure it works before you buy it.
I have some files going back 15 years or so, so I have both my internal drives backed up to multiple external drives and now that online backup is getting cheap I may do that as well.
Do you have a lot of stuff already on your external drive?
If not. I wold copy what you don't have on you hard drive, to your hard drive and format the external to make sure you get all your space back and that it's clean, then try again.
The other thing that I would recommend it that you just copy all of your important files to the external drive.
I have one drive that doesn't have any backups on it, just all my data copied with no compression so that they are always accessible no matter what my backup software is doing.
If I have to reset my computer and install Windows and all my software again it's a pain, but not like losing my files.
Mike