Cannot get USB qube with keyboard to work

[apparatus]

You can use sys-usb only if you have multiple USB controllers available so you can dedicate one of them to dom0 to connect your external USB drive to it.
Then you can hide all other USB controllers from dom0 and use them for sys-usb:

Broken system after installing USB qube

Once you have identified the controllers, you can hide one of them from dom0 - not the one to which your USB device is attached, obviously. Try interrupting the boot and editing the command line to include: rd.qubes.hide_pci 00:1a.00 , or whatever your controller is. If that works as expected, then you can make the change permanent by editing the grub config. This will ensure that one controller is used from dom0, and the other can be safely allocated to sys-usb and used there.