How to Factory Reset Your iMac Without a CD
By Dan Stone
Updated August 29, 2017
Performing a factory reset on an iMac can be a bit involved because they don't have a built-in disc drive. Even if you have an old iMac with an OS reinstallation DVD, it won't do you any good unless you have an external optical drive connected to the system. The iMac features two methods to wipe all hard drives and reinstall the operating system: One uses the built-in recovery partition and the other uses the OS image on a USB flash drive. Both factory-reset methods require an active broadband internet connection.
How to Factory Reset an iMac Via the Recovery Partition
Restart the iMac and hold the "Command-R" keys while the computer reboots.
Select and connect to a Wi-Fi network from the list if the iMac prompts you to do so.
Choose the "Disk Utility" option from the list and click "Continue."
Select the hard drive on which Mac OS X is installed from the tree list on the left side of the window.
Open the "Erase" tab. Select "Mac OS Extended (Journaled)" from the "Format:" drop menu and enter a name for the hard drive in the "Name:" text entry field.
Click "Erase..." to clear all of the hard drive's contents.
Choose the "Quit Disk Utility" option to exit the disk formatting tool.
Select "Reinstall OS X" from the menu, click the "Continue" button and follow the on-screen instructions to complete the OS installation process on your clean hard drive.
How to Factory Reset an iMac Via Bootable USB Recovery Drive
Download the version of Mac OS X the iMac is running from the App Store.
Open the "Applications" directory in Finder. Control-click on the "Install Mac OS X" icon and choose the "Show Package Contents" icon. Expand the "Show Package Contents" folder.
Copy and paste the "InstallESD.dmg" file to an easily accessible location on the iMac hard drive.
Open the "Disk Utility" within the Utilities folder under Applications and connect the USB flash drive to one of the ports located on the back side of the iMac.
Choose the flash drive from the left-side tree display and then click the "Burn" icon.
Select the "Image..." button, then locate and choose the "InstallESD.dmg" file.
Click "Restore" to create the restoration drive.
Restart the iMac after the restore drive is completed and hold the "Command-R" keys until the boot selector appears.
Choose the flash drive from the boot selector, open the "Disk Utility" and select the iMac's hard drive.
Select the "Erase Tab," set the "Format:" value to "Mac OS Extended (Journaled)" and then click the "Erase" button.
Quit the "Disk Utility" tool when the hard drive format is complete.
Choose the "install Mac OS X" option from the menu and follow the on-screen instructions to complete the factory reset.
The format process will erase all information on the hard drive. Copying the OS X installer to a flash drive will erase all contents on the flash drive as well: Back up any important data before reinstalling the OS.
These instructions are based on iMacs running OS X Lion and may vary with other versions of the Mac OS X operating system.
- Apple: iMac
- Apple: iMac Specs
- Apple: OS X Mountain Lion: Erase and Reinstall OS X
- Mashable: How to: Do a Clean Install of OS X Lion
- Apple: OS X Mountain Lion: Reinstall OS X
- Apple: OS X: About Recovery Disk Assistant
- OSXDaily: How to Clean Install OS X Mountain Lion
- OSXDaily: How to Make a Bootable OS X 10.8 Mountain Lion USB Install Drive
- Apple OS X Mountain Lion: The Mac App Store
- The format process will erase all information on the hard drive. Copying the OS X installer to a flash drive will erase all contents on the flash drive as well: Back up any important data before reinstalling the OS.
Dan Stone started writing professionally in 2006, specializing in education, technology and music. He is a web developer for a communications company and previously worked in television. Stone received a Bachelor of Arts in journalism and a Master of Arts in communication studies from Northern Illinois University.