Partition MBR External Hard Drive (USB) when Disk Utility Can’t

On OS X El Capitan and macOS Sierra, you can’t partition an external hard drive which is M.B.R formatted. However, you may risk erasing the external hard drive to GUID partition map or run Stellar Partition Manager software to manage an external hard drive.

If your external USB hard drive is MS-DOS formatted then Disk Utility (OS X El Capitan 10.11 and macOS Sierra 10.12) won’t allow you to add a partition, resize or delete partitions on the external hard disk drive. The reasons behind Disk Utility not enabling you to partition an external hard drive formatted with file system other than HFS is that these drives have “Master Boot Record” partition scheme which is commonly used by the Windows PCs. To perform partitioning actions on such external USB hard drives, you require to re-format the external storage media to GUID partition table. Only when the external hard disk is formatted with OS X Journaled or GUID partition table, the Disk Utility will allow to add, resize and delete partitions on the external hard drive.

Case 1) Task buttons greyed out

In the below screenshot you can see that the important tasks button such as Add (+), Remove (-) and Apply are disabled. One can neither add a new partition or resize, delete another partition. However, re-formatting the external hard drive to OS X Journaled from MS-DOS and EXFAT is allowed.


Case 2) Partition button is disabled 

On selecting the partition from the parent hard drive, the Partition tab is greyed out or disabled.


Solution – a rough approach 

The solution to partition the external hard drive conveniently requires reformatting the entire hard drive to GUID partition i.e. OS X Journaled. Once the external hard disk gets erased to GUID partition map, the Disk Utility will allow all the actions to be performed without any hassle.

GUID Partition 

Now having the drive formatted with GUID (OS X Journaled) partition map, the Disk Utility have all its important tasks button activated for adding, resizing and deleting partitions. Kindly see the below screenshot:


In the above screenshot, it is clearly visible that the GUID formatted external USB hard drive can add another partition. And not just add, but it can resize the hard drive as per convenience. In other words, the Add (+), Remove (-) and Apply buttons are enabled for action.

Erasing the external hard drive to OS X Journaled is recommended for partitioning and resizing the hard drive. However, it is not always feasible to format an MBR external hard disk drive just to create or resize partitions. Erasing the external hard drive will cause the loss of data. Alternative, you can free download and activate Stellar Partition Manager software DMG to perform the tasks of adding a new partition, resizing partition sizes and even removing the partition without reformatting or erasing the external hard drive to GUID on Mac.

How is it done? 

Below is the user-interface of the Stellar Partition Manager application that has ‘Common_Disk’ listed in the left-hand pane of it. This is the same external USB hard drive that we have shown up in above 2 cases.


  1. Size occupied by the Common_Disk external hard drive
  1. Available free spaces
  1. Partition-edge allows resizing the external hard drive. You can increase and decrease the size of the partition with the help of the partition edge. Note: Increasing the partition size is possible only when sufficient amount of free spaces is available.
  1. Add a new partition
  1. Erase the partition
  1. Remove the selected partition
  1. Start the process

Disk Utility vs. Partition Manager

Disk Utility

  1. Requires erasing the MBR partitioned the external hard drive to GUID partition map.
  2. Doesn’t organises free spaces for adding new partitions or resizing the existing partitions.
  3. Can Mount and Unmount a partition.
  4. Cannot work on NTFS or BootCamp partition.
  5. Can’t create a bootable disk for the running OS X 

Partition Manager

  1. Doesn’t require re-formatting of the external hard drive.
  2. Successfully uses the available free spaces on the external hard drive to add a new partition and perform efficient resizing.
  3. Can Hide and Reveal a partition from unauthorised access.
  4. Support adding, resizing and removing NTFS file systems. Supports resizing BootCamp partition.
  5. Can create a Bootable USB for startup disk

Indeed, the Stellar Partition Manager has much to offer when Disk Utility does nothing to partition your MBR formatted external USB hard disk drive. The available features of the partition manager software allow all of the partitioning tasks to be successfully implemented on an internal Apple hard drive i.e. Macintosh HD.