Before selling your second-hand Macbook, the first thing you need to do is wipe your personal data on the device, to avoid possible bad cases. Refer to how to wipe data on Macbook below of Software.
1: Mac Backup
If you just bought a new Mac, you can transfer all the data on your old Mac there, or use a Mac backup using Time Machine to make sure your important data stays safe. You can refer to how to
2: Sign out of all accounts
The software that you have installed on your Mac, will be licensed to you, i.e. the user acquiring your Mac may not transfer the license, except for the operating system. To avoid problems, log out all accounts, your personal information is connected to the Mac.
How to Sign Out of iTunes
Step 1: Open iTunes on Mac.
Step 2: Click Account on the Menu bar in the left pane of the screen.
Step 3: Click Sign Out.
How to sign out of iMessage
Step 1: Open Messages on Mac.
Step 2: Click Messages in the Menu bar in the top left corner of the screen.
Step 3: Click Preferences from the drop-down menu.
Step 4: Select your iMessage account.
Step 5: Click Sign Out.
How to sign out of iCloud
Step 1: Click the Apple logo in the top-left corner of the Mac screen.
Step 2: Click System Preferences from the menu.
Step 3: Click iCloud.
Step 4: Uncheck Find My Mac.
Step 5: Enter the system password when asked.
Step 6: Click Sign Out.
Step 7: Click Remove data from this Mac when asked.
3. Reformat the hard drive
After backing up important data and having logged out all accounts on your Mac, you can now wipe the data on your Mac by reformatting your hard drive.
Step 1: Open or restart Mac.
Step 2: Press and hold the Command + R keys simultaneously during Mac startup. Release your hand from the keys when you see the macOS Utilities window.
Step 3: Select Disk Utility.
Step 4: Click Continue.
Step 5: Select Startup disk (usually named Macintosh HD or similar).
Step 6: Click Erase in the top corner of the Disk Utility window.
Step 7: Select Mac OS Extended (Journaled) from the drop-down menu next to Format.
Step 8: Select the GUID Partition Map from the menu next to Scheme.
Step 9: Click on Erase.
Step 10: After the process is complete, select Quit Disk Utility from the Disk Utility menu at the top left corner of the screen.
4. Reinstall your Mac Clean
After you have reformatted your hard drive, the next step is to reinstall your mac. Just go to the Utilities window. Restart your Mac, then during the Mac startup process, press and hold the Command + R keys until you see the Apple logo.
Note:
Make sure your Mac is connected to the Internet for software verification. During installation, you will receive this message.
Step 1: Open or restart your Mac.
Step 2: Press and hold Command + R while your Mac starts up until you see the macOS Utilities window, then release your hand from the keys.
Step 3: Click Reinstall macOS (or Reinstall OS X) to reinstall the operating system on Mac.
Step 4: Click Continue.
Step 5: Select your hard drive (be it Machintoch HD or similar) when required.
Step 6: Click Install to install the operating system on your Mac. Your Mac will restart once the installation is complete.
Step 7: Once installation is complete, press and hold Command + Q. Skip the setup instructions.
Step 8: Click Shut Down to shut down your Mac.
With MacBook Pro with Touch Bar
Follow the steps below to wipe data stored in the Touch Bar on your Macbook Pro. Or if you use a 3rd-party program or Target Disk mode to wipe the data on your Mac, you can also follow the steps below to make sure the data stored in the Touch Bar has been wiped.
Step 1: Open or restart your Macbook Pro.
Step 2: Press and hold Command + R while your Mac starts up until you see the macOS Utilities window, then release your hand from the keys.
Step 3: Click Utilities in the Menu bar on Macbook Pro, located in the top left corner of the screen.
Step 4: Click Terminal from the Utilities menu.
Step 5: On the Terminal window, enter the following command:
xartutil –erase-all
Step 6: Press Return.
Step 7: Enter Yes when asked to confirm.
Step 8: Click Terminal on the Menu bar, located in the bottom left corner of the screen.
Step 9: Click Quit Terminal.
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By wiping the data on your Macbook that Software has just instructed above, you can apply this way to erase your personal data before selling your old Macbook. Good luck.