I'm not too sure what you mean. I have a MyBook 320 GB 2.0 via 2.0 USB. I connect it to vista, and I hear a little ding sound, but it's not showing up anywhere.
I'm using Leopard (10.5). If you using Tiger (10.4) I believe you will see the same things. If you are using an OS X older than 10.4 then I don't know what you will see.
Plug in your External Drive.
Go to Disk Utility (found in the Utilities folder, in Applications), and open it up. You should see a white vertical box on the left with icons that look like hard drives. One of them should say Macintosh HD. Others may start with a number (2 or 3 digits plus decimal eg 80.2 GB ST350063NS). These are the drives, volumes, etc that the Mac can see. Clicking on them won't hurt anything.
If you see an Icon called Macintosh HD, that is your internal one. Click on it, and look at what it says at the bottom of the window, at the line called "Format: " This is the line that appears under "Mount Point". It will likely say something like Mac OS Extended (Journaled).
Now click on the icon that you think might be the external drive. What does it say next to the "Format" line at the bottom of the window?
Telling us that will help with the next step. Don't click on the other buttons unless you know what you are doing. Just look.
Good Luck