Anyway, now the USB stick is not indispensable any longer. It is needed only if you can't access the media where the backup are stored. That's why, if I were you, I would not use the HDD but and and external device, like a NAS or a USB disk or stick.
Because if the image does not start, you won't be able to access your disk even by FTP.
In my case, I use a NAS and it's independent from the VU+, of course.