Alternate to Disk Utility to Resize an Existing Mac Volume

Disk Utility allows you to resize existing volumes without losing data, but there are a few limitations. Disk Utility can decrease the size of any volume, but it can only increase the size of a volume if there’s enough free space available between the volume you wish to expand and the next partition on the drive.

For practical purposes, this means that if you want to increase the size of a volume, you may need to delete the partition below that volume. You will lose all data on the partition you delete (so be sure to back up everything on it first), but you can expand the selected volume without losing any of its data.

Process to Increase the size of a Volume

1. Launch Disk Utility, located at /Applications/Utilities/.
2. Current hard drives and volumes will display in a list pane on the left side of the Disk Utility window. Hard drives are listed with a generic disk icon, followed by the drive’s size, make, and model. Volumes are listed below their associated hard drive.
3. Select the hard drive associated with the volume you wish to expand.
4. Click the ‘Partition’ tab.
5. Select the volume listed immediately below the volume you wish to expand.
6. Click the ‘-’ (minus or delete) sign located below the Volume Scheme list.
7. Disk Utility will display a confirmation sheet listing the volume you are about to remove. Make sure that this is the correct volume before taking the next step.
8. Click the ‘Remove’ button.
9. Select the volume you wish to expand.
10. Grab the right-hand bottom corner of the volume and drag to expand it. If you prefer, you can enter a value in the ‘Size’ field.
11. Click the ‘Apply’ button.
12. Disk Utility will display a confirmation sheet listing the volume you are about to resize.
13. Click the ‘Partition’ button.

Disk Utility will resize the selected partition without losing any of the data on the volume.If a user is not comfortable with inbuilt disk utility of Mac , then he may opt for some other disk management tool like Stellar Partition manager.
The Stellar Partition manager provides its users the option to:

1. Create new partitions
2. Hide Partitions
3. Merge Free spaces to create partitions
4. Resize Mac partitions

Here is a screenshot to show the options available with Stellar partition manager.