Eventually you might realize that you're running low on capacity, or your drive might die. Western Digital would prefer that you just buy another USB drive, but I'd rather reuse the existing hardware, especially since I have a few larger PATA drives sitting around now. This instructable gives you a play by play on opening the enclosure without breaking it (well, hopefully you won't break it because I'll show you how).
Step 1: Remove the Sides
Pry up the rear end of one of the long silver sides.
Slide the side toward the back of the drive. On my drive it was very difficult. I found that I was able to get things to move by standing the drive on one side and applying downward pressure on the side that I was trying to slide. This relieves some of the stress on the tabs that hold the side in place. Keep trying and be patient, or else you may end up with some broken tabs (like I did, on my first try).
Repeat for the other side.
Step 3: Remove the Top
Step 4: Remove All Fasteners
Step 5: Install Your New Drive, and Reverse the Steps
You should test that everything works after reconnecting the drive, before you go and put it all back together. I found that my drive would not be recognized when the jumper was set to master. By setting it to Cable Select everything worked fine. This is probably a function of how flaky the USB interface is, so YMMV.