If anyone wonders how to access the USB drive (which you can actually read/write into) other than FTP, just plug in a USB storage device, type in \\yourNASipaddress (like \\192.168.1.123) in the address bar of Windows's File Explorer and look for the USBDisk1_1 folder.
But anyway, USBDisk1_1 cannot be modified in Network Shares, ergo, anyone can access it.