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Formatting your MacBook's hard drive will erase all of the data on it. Back up any documents, pictures, or other important files before formatting. You can copy the files to an external drive, burn them to a blank DVD, or upload them to your iCloud storage or other cloud storage service. You'll need an internet connection in order to download the necessary files during the format and reinstall process. ⌘ Cmd+R as soon as it starts up. This will load the Recovery menu. This will open a new window, with all of your drives and partitions listed in the left frame. Select your startup disk to erase the partition that contains your operating system files. You can use the size of the partition to help judge the correct one. If you're getting ready to get rid of your Mac, you'll want to adjust the security options in order to securely erase the data on the drive. You may be presented with several different options, or a slider from "Fastest" to "Most Secure." Select the "7-Pass Erase" or "Most Secure" option to securely erase your data without any chance of retrieval. This will dramatically increase the amount of time the format process takes. This is the format required to install OS X on the Mac. The drive will begin the formatting process. If you didn't change the security options, it will only take a moment. If you chose a secure erase, it may take several hours to complete. " You'll need to be connected to a wireless network in order to proceed with the reinstallation. Follow the prompts to install OS X back on the computer so that you can use it again. The setup program will download all of the essential files automatically.

Summary:
Back up your important data. Ensure your computer is connected to the internet. Reboot your Mac and hold . Select the "Disk Utility" option. Select the partition that you want to format and then click the "Erase" tab. Click the "Security Options" button if you want to securely wipe your data (optional). Select "Mac OS Extended (Journaled)" from the "Format" menu. Click the "Erase" button. Close the Disk Utility window and click "Reinstall OS X.