Disk Utility can Create, Delete, Expand and Trim partitions on OS X 10.11, 10.10, 10.9, 10.8, 10.7 and below versions. However, Disk Utility can't merge partitions into a single hard drive volume without data loss. Actually no other utility or partition manager software can merge all partitions to a single drive. Deleting a partition would cause data loss and Disk Utility may not able to resize the partition effectively. This will again result in loss of valuable unused spaces.

Caution: If you are about to merge partitions with disk utility then consider backing up all the data with Time Machine application. Either use Carbon Copy Cloner to backup all the hard drives data before merging.

Merge With Disk Utility

  1. Select the partition you want to merge and click on "-" button. For instance, you have 3 partitions namely - Macintosh HD, Volume 1 and Volume 2. Then first select Volume 1 and click on the minus sign (-) to remove this partition. A warning message will pop-up. Click Remove to get rid of Volume 1.
  2. Once Volume 1 is removed, resize Macintosh HD to takeover the spaces left by Volume 1. Now select and remove Volume 2.
  3. Again resize Macintosh HD to take over unused spaces left by Volume 2. Click Apply to make changes. Once done, the hard drive will restore to a single volume namely - Macintosh HD.
Problems: The above process may results in inefficient resizing of hard drive thus leaving unused spaces behind.

Merge with Stellar Partition Manager utility

To overcome the problem of inefficient resizing by Disk Utility you can take help of Stellar Partition Manager utility:

  1. Create bootable media with this utility and boot the Mac
  2. Remove Volume 1 and Volume 2 partitions and resize the Macintosh HD to claim all the left spaces. The software collects all the free spaces and organizes them at the bottom of the partition. This feature isn't available in Disk utility which results in unproductive resizing
  3. Once Macintosh HD is resized click on software Apply button to save all changes. Now Mac is ready with single volume named - Macintosh HD
Either ways are helpful in merging the partitions to a single drive however both the methods results in data loss. You should only move forward with merging once proper data back is ensured. Otherwise crucial data from the partitions will be lost.