How to Format an iMac

By Christopher Capelle

Updated February 10, 2017

i Kevork Djansezian/Getty Images News/Getty Images

The iMac is Apple’s all-in-one computer—with the display, drives and all components (except input devices) neatly packaged in a slick-looking desktop computer. There are many reasons to want to format the hard drive in your iMac—if the hard drive is giving you errors, reformatting it may solve the problems. If you’re planning to give it away or sell it, formatting will erase your personal data and give the next user a clean slate. No matter which model you’re working on, formatting the unit is fairly simple, and the process is virtually the same with all the different models of iMac.

Formatting With Installation Disc

Back up all personal data from the iMac before proceeding. Formatting the drive destroys all data, so confirm your data is backed up beforehand. Back it up using an external hard drive, CDs or DVDs, or a flash drive. If you have an Apple OS installation disc, then follow Steps 2 through 5. If you don’t have the installation disc, then simply skip ahead to Section 2, "Formatting Through Target Disk Mode."

Insert the OS X installation disc into the computer’s drive and then restart the iMac by selecting “Restart” under the blue apple in the upper left corner of the screen. Once the reboot chime sounds, hold the “C” key down and allow the computer to boot from the installation media.

Click on the right-pointing arrow in the lower right corner of the installation wizard window to begin the process. Select “Agree” when the legal window appears. After that, you’ll be asked what kind of installation you want to perform.

Click on the Options button to choose your installation type. Select the button for "Erase and Install,” which is the choice at the bottom of the list. Click on “Install,” and you will get a final warning about formatting the drive. Opt to format the drive.

Allow the erase and installation process to complete and reboot your computer when prompted. Once the iMac reboots, you’ll see a window that allows you to begin the setup process. If you plan on selling this iMac or giving it away, unplug it from the power source at this point. That way, when the next user boots it up, he’ll be able to name the computer and establish a user account exactly the way he wants to. If you are reusing the iMac or planning to set it up for somebody else, complete the setup wizard by following the prompts.

Formatting with Target Disk Mode

Attach your iMac to another Mac with a FireWire cable.

Reboot the iMac while holding down the “T” key, which boots the iMac in FireWire mode. Once the FireWire symbol appears on the display, open Disc Utilities (Applications>Utilities) on the second Mac, and highlight the iMac’s drive in the left.

Select the “Erase” tab and click on the “Erase…” button. The iMac’s hard drive will then be formatted.

Items you will need

  • Mac OS X installation media

  • FireWire cable


Use the original OS media if possible.


All data on the computer will be destroyed when the hard drive is formatted, so back it up beforehand.