In some circumstances, it can happen that a USB device lacks a dedicated device driver; in this case, a GNU/Linux system simply enumerates it, and then no driver is loaded at all. In this situation, the user cannot get access to the new USB device in any manner, except by accessing it using the raw commands directly over the bus. Simply speaking, it directly sends the USB messages to the new device and then manages the answers without using any dedicated driver at all.
You should note that if this new device has no available driver, then it cannot be seen from the system as any usual device (for example, a keyboard or a storage disk), so we have no
/dev/sdb entries to use. However, even if this situation may appear quite strange and difficult, in reality, it's not so terrible. In fact, for very simple devices, we can implement a simple management code in the user space using the
libusb library on the host PC.