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.