Off course you can do that.

But I have had problems coneecting USB-IDE drives to knoppix. I had one USE 1.1-IDE enclosure and wasn't able to recognize it under knoppix. I replace that case with a shiny new USB2, And the new one works perfectly under knoppix.

If your case doesn't work, you can try to connect your spare hdd inside the box. In this case you will 100% sure thay knoppix will recognize it.