If you want your Mac to start up again from the volume you just you erased, select Reinstall macOS in the utilities window, then click Continue and follow the onscreen instructions.When done, quit Disk Utility to return to the utilities window.Optional: If you previously used Disk Utility to add internal volumes other than Macintosh HD, you can erase them individually using the same process.If this button isn't shown, click Erase instead. An internal hard drive, as the name suggests, is a drive built into the device. In my case, I even have programs I want to keep so I don't have to re-download them after the process is over. Reformatting different types of hard drives The steps above are the same whether the hard drive you’re formatting is internal or external. Step 1: Back Up Your Files Okay, so you may have a lot of files that you want to get rid of, but you definitely have some files you want to keep. Format: APFS or Mac OS Extended (Journaled), as recommended by Disk Utility Select the file system you want to use (default is APFS) and click Erase.Click the Erase button in the toolbar, then enter the requested details:.Select Macintosh HD in the sidebar of Disk Utility.From the utilities window, select Disk Utility and click Continue.If asked, select a user you know the password for, then enter their administrator password.Start up from macOS Recovery: Turn on your Mac, then immediately press and hold these two keys until you see an Apple logo or other image: Command (⌘) and R.

Reformat mac hard drive