IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
After you attach and mount the external drive, chose GO > UTILITIES in the Finder, and when the /Applications/Utilities folder opens, launch the Disk Utility. Look in the sidebar on the left and find and click on the icon for your external HDD. Click the erase tab at the top of the window, then from the Volume Format menu choose the proper format, Mac OS Extended or whatnot. Choose a name for your external HDD and type it in the Name field. Click erase and sit back and wait for the magic. That should pretty much do it.