Hi @fcodeq, and welcome here.
- If @otter2 's solution doesn’t work (I have used it so it looks good to me), you can create one or more app qube that uses the whole HDD. Then, inside that qube you can create volumes (I.e.: loop devices) that your regular qubes will access and mount.
- I personally use both solutions. I use the Qubes Backup tool to backup some essential qubes (templates and app qubes) to be able to restore my system quickly. I also use another program to backup data, from the qubes. And I use Salt to record most of the configuration of the system.