How To Delete OSX Mountain Lion (or Other OSX) Boot Partition – Recently, Apple has just pushed out the third Developer Preview edition of OS X 10.8 codename Mountain Lion. Obviously there are bunch of bug fixes and performance enhancements there. But however, there is always a reason to remove it. And if you have installed OSX 10.8 DP3 in dualboot mode on your current stable OS X like Lion or Snow Leopard, then read this on to know how to uninstall Mountain Lion and delete its boot partition.
For just in case, you better make backup of your Mac before proceeding these steps..
Steps To Remove OSX 10.8 Boot Partition On OSX 10.7.x Lion
1- Logon to OS X Lion
2- Launch up Disk Utility
3- Now select the primary hard drive
4- Click on “Partition”
5- Select the “Mountain Lion” partition
6- Then click the [-] button to delete the partition
7- Click on “Remove” button to confirm
8- Quit Disk Utility
9- Reboot Mac OS X
10- But during boot, hold down the Option key
11- From the Boot menu, select “Recovery”
12- Open Disk Utility again
13- Select the hard drive
14- Click on the “Partition” tab
15- Click and drag the partition resizer all the way to the bottom
16- Then click “Apply” and “Partition” to confirm resizing
17- Finally, reboot your Mac OS X as usual
18- That’s it.
Once your Mac reboots, the previously OSX Mountain Lion disk partition will now be allocated back to your primary OS X.