移動硬盤 is the one with a HD inside a external enclosure as a whole package, with either Firewire, USB2 or both connections.
Mobile Rack is a kind of internal enclosure, you install the base Rack on your computer like a CD-Rom. You install a HD in a tray, that tray then can be inserted into the rack to connect to the computer, if you have same type of Racks installed into different computers, you can transfer data between with the HD in the tray.Otherwise, the HD becomes a backup HD.
External Enclosure is a box with Firewire, USB2 or both. You can install a HD, or even CD-Rom in it. It's like 移動硬盤 without the HD. But it's your choise which HD you want to use.