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You can do this by using one of your computer's USB-C ports. In order to back up your Mac, you will need an external hard drive on which to store the backup. Your external hard drive must be formatted for use with a Mac for this to work. You can also use a high-capacity flash drive for this step. If your selected external drive uses a USB 3.0 connection, you'll need a USB 3.0 to USB-C adapter in order to connect the drive to your computer. Click the Apple logo in the top-left corner of the screen. This will prompt a drop-down menu. It's near the top of the drop-down menu. This app icon resembles a green clock dial. The Time Machine window will open. It's near the top of the page. A window will open. Click your external drive's name in the window. It's at the bottom of the window. Doing so will set the external drive as your Mac's Time Machine backup drive. It's the clock dial-shaped icon in the upper-right corner of the screen. A drop-down menu will appear. You'll see this near the top of the drop-down menu. Doing so prompts Time Machine to begin backing up your Mac to the external drive. You'll see a "Time Machine has finished..." message once the backup finishes, at which point you can eject and remove the external hard drive.
Connect an external hard drive to your Mac. Open the Apple menu . Click System Preferences. Click Time Machine. Click Select Backup Disk. Select your external drive. Click Use Disk. Click the Time Machine menu icon. Click Back Up Now. Wait for your backup to complete.