You can’t format the hard disk on which Windows is stored while Windows is running, for the same reasons that you can’t work on a car’s engine while you’re driving it. To format the primary drive - the hard disk where the operating system resides, also called C drive or C: - you need to boot into the setup program on Windows 7 or Windows 10 and use it to format your drive. The following steps show you how to format your primary drive when all applications and your files are gone. Then you can use the Windows setup program or macOS Recovery Environment to format primary hard drives.

You may need to reinstall the operating system, but you don‘t want to lose your data! If it’s at all possible, begin by backing up and rescuing everything of importance, or clone your hard drive by using one of our guides: How to clone a hard drive.

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Format for a mac on windows