These instructions apply only to Mac computers with an Intel processor.
Erasing your Mac permanently deletes all of its files, so make sure that you make a backup of any files that you want to keep. If you're attempting to restore your Mac to factory settings, first learn what to do before you sell, give away, or trade in your Mac.

RELATED: 8 Mac System Features You Can Access in Recovery Mode. Your Mac’s Recovery Mode is a treasure trove of useful tools, and it’s the easiest way to wipe your computer and start from scratch. Shut down your Mac, turn it on while holding down Command+R. Your Mac will boot into the recovery partition. Disk Utility will automatically choose the format for you. This may be APFS, or it may be Mac OS Extended (Journaled) depending on how the drive is currently formatted and whether you using macOS. File system formats available in Disk Utility on Mac. Disk Utility on Mac supports several file system formats: Apple File System (APFS): The file system used by macOS 10.13 or later. Mac OS Extended: The file system used by macOS 10.12 or earlier. MS-DOS (FAT) and ExFAT: File systems that are compatible with Windows. Open Disk Utility for me. Use macOS Internet Recovery to boot your Mac. Instructions here - How to Start Up a Mac in Internet Recovery Mode. Once it connects to the internet and boots into recovery, you can access Disk Utility from it. Use the Disk Utility to format the new drive. Then you can use your bootable flash drive to install macOS Catalina to the new drive. When using Disk Utility 16.3 with macOS Sierra to format a 'thumbdrive' USB 3 Lexar flash drive (64 gigs), when I choose a Format of ‘Mac OS Extended (Journaled)’, I get a list of three Scheme item.

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. You may then be asked to select a user you know the password for. Select the user, then click Next and enter their administrator password.

Use Disk Utility to erase your disk

1. From the utilities window in macOS Recovery, select Disk Utility and click Continue.
   
2. Select Macintosh HD in the sidebar of Disk Utility. Don't see Macintosh HD?
   
3. Click the Erase button in the toolbar, then enter the requested details:
   • Name: Continue to use Macintosh HD as the name, or enter a different name.
   • Format: Continue to use APFS or Mac OS Extended (Journaled), as recommended by Disk Utility.
4. Click Erase Volume Group. If you don't see this button, click Erase instead.
   
5. If Find My is turned on for this Mac, you may be asked to enter your Apple ID to authorize the erase. Forgot your Apple ID?
6. After the erase is done, select any other internal volumes that may be listed in the sidebar, then click the delete volume (–) button in the toolbar to delete each volume.
   You don't need to delete the Macintosh HD volume you just erased, or any external volumes or disk images.
7. Quit Disk Utility to return to the utilities window.
8. If you want to start up again from the disk you erased, you should now reinstall macOS on that disk.

If you don’t see Macintosh HD in Disk Utility

Your built-in startup disk should be the first item listed in the Disk Utility sidebar. It's named Macintosh HD, unless you changed its name. If you don't see it there, choose Apple menu  > Shut Down, then unplug all nonessential devices from your Mac and try again.

If your disk still doesn't appear in Disk Utility, or Disk Utility reports that the erase process failed, your Mac might need service. If you need help, please contact Apple Support.

Learn more

For more information about using Disk Utility, see the Disk Utility User Guide.

If you’re still using a Mac OS X the time will come when your computer won’t boot, or a problem may arise where you can’t take control of the device, and booting from an OS X installation media will be required.

This is why it’s recommended that you make a Mac OS X bootable USB when your Mac is in working conditions. However, if you find yourself on a scenario where your device (iMac, MacBook Pro, Air, Mac Pro or Mini) is not responding and you happen to have a Windows 10 device, then you can still be able to make a USB bootable installation media for your Mac OS X to reinstall the operating system using the Recovery Assistant.

These instructions will also work for Windows users who are running Mac OS X on a virtual machine and need to upgrade to the latest version. For instance, to OS X Yosemite.

Requirements

Before you dive into this guide, you’ll need a few things:

• A broken Mac computer with Mac OS X.
• A trial copy of the TransMac software.
• One high quality USB flash drive with 16GB of storage.
• A copy of Apple’s macOS (DMG file).

Now that you have all the necessary ingredients, you’re ready to make a Mac OS X bootable USB using the DMG file of the operating system with the steps below.

Create Mac OS X bootable USB installation media

Before you can use TransMac, you first need to partition your USB flash drive with a GPT partition, as a normal MBR partition may not work. To do this, you’ll need to use the Diskpart command-line utility on Windows 10.

Setting up GPT partition

Use these steps to set up a USB drive with a GPT partition:

1. Open Start on Windows 10.

2. Search for Command Prompt, right-click the top result and select the Run as Administrator option.

3. Type the following command to open Diskpart and press Enter:

4. Type the following command to determine the USB flash drive and press Enter:

5. Type the following command to select the storage and press Enter:

   Quick tip: The select disk 1 command as an example, but you have to replace 1 with the number of the flash drive you want to use.

6. Type the following commands to delete everything from the USB thumb drive and press Enter:

7. Type the following command to convert the drive into a GPT partition and press Enter:

8. Type the following command to select the new partition and press Enter:

Mac Disk Format Types

After you complete the steps, the USB flash drive from MBR to GPT format, you can use the steps below to create a bootable USB installation media to install Mac OS X.

Create USB install media

Format Flash Drive Mac Os Extended Journaled In Windows

Use these steps to create a bootable media to install Mac OS X:

Format Flash Drive Mac Os Extended

1. Download and install a copy of TransMac.

   Quick note: TransMac is a paid software, but it has a 15-day trial solution, that give us more than enough time to move the DMG files to the USB drive from Windows. (If you want to support the developer, you can purchase the full version.)

2. Insert the USB drive that you’ll use to fix your installation of OS X. (Remember that all the data in the USB will be erased. Make sure you take off any important documents.)

3. Right-click the TransMac software icon and Run as administrator. (You’ll be prompted to Enter Key or Run, because we’ll be using it once, click the Run option.)

4. On the left pane, you’ll see all the Windows PC drives listed, right-click the USB drive that you’re intending to use to reinstall Apple’s OS X and select the Restore with Disk Image option.

5. In the warning dialog box, click the Yes button.

6. Use the Restore Disk Image to Drive dialog box to browse for the DMG file with the installation files for Mac OS X Yosemite in this case, and click the OK button to create a bootable USB of the operating system.

   Now, you’ll have to wait a long time. No kidding. It could take one or two hours to complete the process depending on your computer and other variables.

Format Flash Disk Mac Os X

Once your bootable USB installation media is ready, remove it and insert it into your Mac, power it on, holding down the Option key, and select the USB you just created to reinstall Mac OS X.

Format Flash Disk Mac Os Catalina

If you’re having issues trying to create a bootable media, you can get a USB flash drive that comes with Mac OSX ready to install.