So you're essentially giving them a 200GB disk that you don't want them to format and use on their home computers?

From my expirence, USB HDD's are acturally a IDE to USB controller card, you could theoretically grab ahold of the write blocking thing from a forensics kit and place it between the USB controller and the HDD. But there is a catch. I don't think the blocking thing is cheap. And I don't think they are all too tiny either. So you'd potentially be doubling the cost of the drive, and run the risk of not being able to reassemble the enclosure to hide the fact that you made internal modifications.

Plus, if they take the enclosure apart, they're left with a run-of-the-mill IDE hard drive to put into their computer and even have a gadget on the side that could almost let them start their own computer forensics lab...

BTW, I'd ask HogFly about the prices for these write blocking things, and the dimensions, etc, to help decide if this is worth it. Maybe a smaller drive is in order, or a DVD?