The IO Data 40GB HDMX-U40 is a 1.8-inch drive packed into an aluminum enclose for external mobile use. You wont get an external HDD much small then this one. The device measures only 76 x 83.5 x 16.2mm and will connect and receive power via USB 2.0 to any Windows 98, ME, 2000 and XP as well as Mac OS 9.0.x to 10.3.X.

Available in late April for 34,000 YEN, or $310 USD.