If you would like to have an extra set of backups on USB drives in addition to your PBS server then you can:

Attach a USB drive to each Proxmox node.

Create a directory on each node that uses the USB drive. Give it the same name on all of your nodes.

Go to the Datacentre at the top and define a backup job using the new USB drives as the target.

Set the Content to be VZDump backup file, as shownk above, and disable the backups flag on the Backup Retention tab.

In my case I did not use the Shared toggle. If you have a large USB drive I suppose you could just connect that drive on a single node, instead of one USB drive on each node, and share it among all of the nodes. I have not tested this.