MBL does not have a USB port, if yours do, you have a different model and probably wrong forum.
If you do really have a MBL with no usb port, yes, you should be able to backup to a USB drive, but it will need to be attached to a machine or router. Depeding on your setup. In a few words, you will need to make the USB drive available to the network somehow. Not simple and probably for mid-level to advanced users.
The simplies solution, get another NAS, and create safepoints. Check manual for more details.