Disk Utility can't directly erase or format a startup disk, because Disk Utility, and all of the system functions it uses, is located on that disk. If Disk Utility tried to erase the startup disk, it would at some point erase itself, which could present a bit of a problem.
To get around this problem, use Disk Utility from a source other than the startup disk. One option is your OS X Install DVD, which includes Disk Utility.
Using Your OS X Install DVD
- Insert the OS X Install DVD in your Mac's SuperDrive (the CD/DVD reader).
- Restart your Mac by selecting the 'Restart' option in the Apple menu. When the display goes blank, press and hold the 'c' key on the keyboard.
- Booting from the DVD can take a little time. Once you see the grey screen with the Apple logo in the middle, you can release the ‘c’ key.
- Select ‘Use English for the main language.' when this option appears, then click the arrow button.
- Select 'Disk Utility' from the Utilities menu.
When Disk Utility launches, follow the steps outlined in the 'Erase a Non-Startup Volume' section of this guide.
Using the OS X Recovery HD
For Macs that do not have an optical drive, you can boot from the Recovery HD to run Disk Utility. Starting Up From the OS X Recovery HD Volume
Restart Your Mac
- Quit Disk Utility by selecting 'Quit Disk Utility' from the Disk Utility menu item. This will take you back to the Install OS X window.
- Quit the OS X Installer by selecting 'Quit OS X Installer' from the Mac OS X Installer menu item.
- Set the startup disk by clicking the ‘Startup Disk’ button.
- Select the disk you want to be the startup disk and then click the ‘Restart’ button.