Move bootable iso from CD/DVD Drive to Hard Disk

I've got my Proxmox VM setup with a CD/DVD drive, USB Device and (small) Hard Disk.
It's set to boot from the USB drive, which holds a bootable ISO disk.
How can I transfer the OS from the USB drive to the Hard Disk? (or even the CD drive).

These are instructions in a non-virtual environment, ...but I can't 'see' the hard disk when I use fdisk in the Console for the server.
https://hiveon.com/knowledge-base/guides/hdd_move/

(I'm super new and learning as I go, pls keep it simple and...

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