How to Remove OS X Mountain Lion (or Any Other Mac OS X Boot Partition)

For those dual booting between OS X Mountain Lion and OS X Lion, or any other two versions of OS X for that matter, there comes a time when you inevitably want to remove one of the operating systems. For this walkthrough we’ll assume the boot partition you want to delete is one of the developer previews of OS X Mountain Lion but it could be any other OS X boot volume as well.

It would be a wise idea to backup your Mac before proceeding, you will be editing the partition map of the drive and there is always a possibility something could go wrong.

From OS X Lion:

1. Open Disk Utility and select the primary hard drive
2. Click on “Partition”
3. Select the “Mountain Lion” partition and click the [-] button to delete the partition



4. Confirm the partition removal and quit Disk Utility
5. Reboot Mac OS X and hold down the Option key during boot, select “Recovery” from the boot menu
6. Open Disk Utility and select the hard drive, again selecting the “Partition” tab
7. Click and drag the partition resizer all the way to the bottom, then click “Apply” and “Partition” to confirm resizing (*see below if you get a “Partition Failed” error)



8. Reboot Mac OS X as usual

* If you encounter a “Partition Failed” error resolve it by running fsck from single user mode:

• Hold Command+S at startup and type “fsck -fy”
• Reboot OS X as usual, then launch Disk Utility to resize the partition

When Mac OS X reboots the partition space that used to be allocated to OS X Mountain Lion will now be allocated back to the primary operating system, OS X Lion.