To resize the drive do the following:
1. Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
After the main menu appears select Disk Utility and click on the Continue button. Select the hard drive's main entry then click on the Partition tab in the DU main window.
2. You should see the graphical sizing window showing the existing partitions. A portion may appear as a blue rectangle representing the used space on a partition.
3. In the lower right corner of the sizing rectangle for each partition is a resizing gadget. Select it with the mouse and move the bottom of the rectangle upwards until you have reduced the existing partition enough to create the desired new volume's size. The space below the resized partition will appear gray. Click on the Apply button and wait until the process has completed. (Note: You can only make a partition smaller in order to create new free space.)
4. Click on the [+] button below the sizing window to add a new partition in the gray space you freed up. Give the new volume a name, if you wish, then click on the Apply button. Wait until the process has completed.
You should now have a new volume on the drive.
It would be wise to have a backup of your current system as resizing is not necessarily free of risk for data loss. Your drive must have sufficient contiguous free space for this process to work.
The mechanics of using Disk Utility to do the resizing, as explained by Prof. Kappy, are right on the money, but that won't work on Lion while using the Recovery Partition on the internal drive. You will get a constant and consistent error that the partition cannot be unmounted. With previous cats it works just fine, cause you would have booted from the Install DVD or CD and weren't using the internal drive at all. So we need to duplicate that condition in Lion as well. So:
- Get an external USB hard drive or thumb drive with at least 1GB of free space. Note that the partition or device will be completely erased, so make sure it does not have important and irretrievable stuff.
- Download the Lion Recovery Disk Assistant from here: http://support.apple.com/kb/DL1433 then follow these instructions to copy Lion's Recovery Partition onto the external device: http://support.apple.com/kb/HT4848
- Restart the Mac while leaving the newly created Recovery Disk plugged in and hold down the Option key before the startup chime.
- In the list of bootable devices that appears, select the icon corresponding to the Recovery Disk and click on the arrow that appears. This will cause the Mac to boot from it and leave the internal drive untouched.
- Now you can follow Kappy's instructions without any problem.
Alternately, for those power users that wisely saved their Lion Installer and created a full Lion Installer Drive (see Kappy's instructions here: https://discussions.apple.com/thread/3786371?tstart=0 ), you can boot from that as well and use the Disk Utility found there.
Either of these two alternatives will allow you to do the resizing without requiring an Internet connection and hoping the Mac chooses to start from Apple's servers instead of the local Recovery Partition.
Just writing to confirm that my experience is that
Courcoul's method does work with Mountain Lion, while Kappy's method does not work with Mountain Lion.
I repeatedly got the
Partition failed with the error:
Couldn't unmount disk.
error until I downloaded what is now called "OS X Recovery Disk Assistant v1.0" and used it.
Booting off the recovery partition, it remained impossible to resize the OS partition on the same drive, despite multiple tries and plenty of free space.