How do I upload files to an external hard drive using terminal?

Question

I'm currently trying to upload some files off of my device onto an external hard drive using Terminal, and I've run into some issues.

Using these steps: https://gist.github.com/kwcto/45e0b985071fad655e17

1. I've run the command diskutil mount /dev/disk1s2, which should allow me to read/write on the hard drive.

2. I used cd /Volumes/<Hard Drive> and ls to navigate onto the hard drive.

3. When running the command mkdir Backup2021 (which should create a folder on the hard drive), I'm met with the error code mkdir: Backup2021: Read-only file system

4. I'm also unable to copy files from my device onto this external drive, as the walkthrough demonstrates.

This issue is constant over different devices; am I doing anything wrong, or is there an alternative route?

I'm mainly running macOS 10.12.6, but this issue has persisted on devices with different operating systems.