An external drive is the same as an internal drive just not installed inside the case. Meaning you can use it in exactly the same way as an internal drive.
A backup drive is usually a zip drive designed to back up you hard drive and isn't used as a hard drive itself. It is usually in the form simalar to a floppy disk that the backup information is copied to. You can then use the backup disk and drive to reinstall all of your information from your original hard drive back to the hard drive in case of a virus or manually removing something that was needed for the system to run.
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usually just capacity. Most flash (or jump or thumb or...) drives are just a couple of gigabytes - whereas external hard drives can be up to (and eventually larger than) a terabyte.
An external hard drive is an actual hard drive that connects to the computer through and USB port. And external backup drive can be a storage unit as to where files are uploaded.