How do I move files to external HD via Terminal?
Based on your question it appears the problem you're having is related to the use of ~ in the external drive file path in the terminal commands you're trying to use. 

Basically, the tilde symbol (~) in macOS command line usage refers to your home directory, e.g /Users/Veronique.

So, if you're trying to run a command such as: cp ~/Desktop/*.jpg ~/Volumes/1 this won't work because it's looking in the wrong place for the drive named 1. Instead, the command you need to use would be:

cp ~/Desktop/*.jpg /Volumes/1


Remember also that if your external drive name has a space in it, then you need to factor this into your command. For example, let's say you're trying to copy a file called ATM.jpg from your user Desktop folder to an external drive called Backup Drive, then your command would be:

cp ~/Desktop/ATM.jpg /Volumes/Backup\ Drive


Note the use of the backslash and space to denote the space in the external drive name.