The drive must be repartitioned. If it's the drive you start up from, you must start up from another physical drive (such as a local Time Machine backup drive) or in Internet Recovery mode.
If your Mac shipped with OS X 10.7 or later preinstalled, or if it's one of the computers that can be upgraded to use OS X Internet Recovery, you may be able to netboot from an Apple server by holding down the key combination command-option-R at the startup chime. Release the keys when you see a spinning globe. Then select Disk Utility in the OS X Utilities screen.
Note: You need an always-on Ethernet or Wi-Fi connection to the Internet to use Recovery. It won’t work with USB or PPPoE modems, or with proxy servers, or with networks that require a certificate for authentication.
This operation will remove all data from the drive. You must have at least two complete, independent backups of all data on the drive that you intend to keep.
- Please select the icon of the drive you want to partition—not any of the volume icons below it—in the Disk Utility window.
- Click the Partition button in the window toolbar.
- Set up the partition scheme you want. Usually the default choice (one data partition) is correct.
- From the Format menu, select Mac OS Extended (Journaled).
- Click the Options button (if there is one) and select GUID Partition Table. You may not see the button.
- Click the Apply button and confirm when prompted.
