dom0 is supposed to be as isolated as possible for security and shouldn’t directly mount partitions and interact with files. I’m not saying there’s never a need to mount an external HDD to dom0, but I’m just posting this in case people don’t know much about handling block devices (disks, partitions, etc.).
Usually the disks (or just partitions of those disks) are given to different qubes (a “qube” is a virtual machine in Qubes) with the GUI or with the qvm-block command.
For example, my movies, music and TV shows I have a media qube that I give a few big partitions full of, well, media files to. “Media” doesn’t have internet access because it doesn’t need it to play files.
Some personal stuff I could put in other qubes. You might have a qube for personal documents, another one with work-related files that has internet access, etc. Each qube could get a few partitions.
Please go over this article as it explains the foundations of handling block devices in Qubes:
If you already know this, at least I hope that this could be useful to others.