Change Permissions on macOS USB Device
I was never able to find a way to get direct access to the raw device, however, VirtualBox has a "raw disk access" feature that allowed me to get around this problem.

The way this works is a VMDK drive is created that is actually mapped to a physical volume.  This acts as a passthrough so that VB can access a physical device.

VBoxManage internalcommands createrawvmdk -filename "&lt;/path/to/file&gt;.vmdk" -rawdisk /dev/disk#


For example, if I had a physical USB device with an OS installer I could create a VMDK which I could attach to a VM.  

Example:

Assume a USB installer with macOS/FreeBSD/Linux/Windows.  To attach that physical USB to a VM (assuming /dev/disk3 is the USB device), 


Insert and unmount (not eject) the device.  

diskutil unmount /dev/disk3

Create the VMDK

sudo VBoxManage internalcommands createrawvmdk -filename "USB_OS_Installer.vmdk" -rawdisk /dev/disk3

Take ownership of the VMDK (it was created as root via sudo; replace "user" with your username and leave the group "staff" as is)

sudo chown user:staff USB_OS_Installer.vmdk

Attach the VMDK to the VM in VirtualBox's GUI. 


You now have access to a physical boot media device through your VM.