Resizing Boot Volume in Mac OS X Involves Risks

"I need to create another volume in my Mac for Windows installation, but lacking free space. Can I trim its boot volume? I am scared of losing the data, does it involve any risk?”It is one of the many practical situations which Mac users often observe. 

Resizing a volume in Mac OS X is not a tough task until or unless you know how to do it, and how to avoid the risks involved. Moreover, the possibility of a Mac volume being resizable relies on the Partition Map Scheme used to build it. You can resize a Mac volume only if it was built by choosing GUID Partition Table (GPT) as the Partition Map Scheme, but not Master Boot Record (MBR) or Apple Partition Map (APM). 

Perhaps you think, just like all other volumes, it is possible to resize Mac boot volume, as it is the most susceptible partition on a Mac machine. Well, it is possible to do the same (following Partition Map Scheme) on a Mac boot volume, but only with the involvement of risks. Hence, this task requires additional user attention, as resizing a boot volume using Disk Utility may disturb its HFS file system resulting in system crash and hence data loss. 

Since the scenario demands discussion about the boot volume in Mac, let us discuss how to resize it safely under this narrow criterion. Go to recovery mode to resize it, as recovery mode ensures for OS security and excludes the risk of data loss.

• Restart Mac and immediately press and hold Option and R keys (or Command key) after you hear startup tone.

• Release the key(s), select Recover HD from the Boot Manager and click the downward pointing arrow.

• Select Disk Utility in the main menu and click Continue button.

• Select the source hard drive and go to the Partition tab in the right pane.

• In the graphical view of the hard drive, select the Macintosh HD and drag it upward from its bottom right corner to reduce its size.

(The blue colored area in the graphical view represents the data saved in it, but not its exact location.)

• Click Apply button to save the changes and restart Mac.

Note: Backup your important data before you resize Mac boot volume because you never know what may happen next moment. It is suggested to go for incremental data backup and keep it at a secure place.