When it comes to external hard drives and portable hard drives there is not much difference between the two hard drives.
Both external hard drives and portable hard drives are external to the computer they are connected too so they can be moved around.
But the regular external hard drive that is larger is usually meant to just sit on a shelf or your desk and connect externally to your computer to backup files too.
The portable hard drive also can have data backed up to the hard drive and plugs in externally as well but the portable hard drive is smaller and lighter weight where you can take the hard drive with you easily and some external portable hard drives are small enough to fit into your pocket.
I have a larger external hard drive on my desk that is almost as big as my desktop computer and that is also portable because I can take it anywhere I want too but it's not meant to be moved around a lot because of it's size.
The smaller portable hard drive I have is also external but I can easily just slide it into my pocket and be on my way.