Flatpak doesn't allow sharing a USB drive with a VM
Using the flatpak of Boxes from Flathub, there's no way to share a USB drive plugged into the host with a VM. Normally this would be under "Devices and Shares" in the Properties dialog, but there's some hole that needs to be poked in the sandbox to make it work.
This issue was originally at https://github.com/flathub/org.gnome.Boxes/issues/2