Create new filesystem:

```
sudo dd if=/dev/zero of=/usb.bin bs=512 count=51200
sudo mkdosfs /usb.bin
```

* Add `dwc2` to `/etc/modules-load.d/modules.conf`
* Add `dtoverlay=dwc2` to `/boot/config.txt`
* reboot
* `sudo modprobe g_mass_storage file=/usb.bin stall=0`