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Make sure the Name section says "Macintosh HD.".Once you have selected the drive, click Erase in the toolbar.If you don't see Macintosh HD or your chosen disk name, shut down your computer, unplug all external devices (you can leave things like mouse and keyboard plugged in if necessary), and start these steps over.Select Macintosh HD in the sidebar underneath the Internal heading (or the name of your disk if you've changed its name from Macintosh HD).Turn on your Mac, and while the Mac is restarting, hold down the keyboard combination cmd+R until you see the Mac logo or another icon or image appear.
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Here's how to reset Macs with Intel chips and reinstall mac OS if you choose to do so:
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