The official mini-B USB connector has five pins, and looks like this:
Unfortunately, there are almost as many unofficial mini-USB connectors as sands in the sea, and the unofficial ones are, for some reason, widely used.
This page lists some of the more common ones (but not all). Basically, you need to get a cable that fits the connector on your drive.
And to think that people bitched about FireWire having a 6-pin and a 9-pin cable.