Click Erase and depending on how much you have stored on it, it could take seconds or minutes to complete.Click the Erase tab at the top of the screen.Select the drive from the left side of the window.Launch the Disk Utility app which can be found in Applications > Other.Plug in your external hard drive or USB flash drive to your Mac computer.The process to format an external hard drive or USB flash drive for your Mac is pretty easy and straightforward. How To Format An External Hard Drive For Mac However, if you’re looking to ensure full compatibility between Windows and Mac, check out the steps below. We’re sure that some of you have encountered situations where you plug in an external hard drive or USB flash drive to your Mac computer, only to find that it won’t read or you can make changes to it due to it being formatted only for Windows. This can make it a bit tricky if you’re a Mac user, especially if you’re looking to buy an external hard drive where sometimes, some models might come preformatted for Windows use. Most accessory makers tend to build accessories for the more dominant platforms, which is why a lot of computer peripherals tend to be more focused towards Windows users.