Disk-Utility-Failure-PartitionThe failure of the Disk Utility occurs due to various reasons and is exhibited in the form of some errors. Let us analyze some of the commonly encountered errors.

1. The first such error is

‘Disc erase failed couldn’t unmount disc’


‘Disk Erase failed with the error: Couldn’t unmount disk.’

This does not allow the repair of directory or permissions in Disk Utility or the erase or partition of the drive.


This type of error occurs when the boot drive gets modified or is being accessed by some other application.


Another bootable drive can be used for starting up Mac like an external drive or an OS X Recovery. Disk Utility is run from there. The preference is for the external drive or a network attached drive because the OS X Recovery may not be able to unmount or modify the same internal disk.

Once booted from OS X Recovery, the ‘diskutil’ and the ‘sudo’ commands are used. If this fails, then the option is the backup of all the files in an external drive or another internal drive and then subsequent removal of everything from the hard drive. We need to force erase the physical disk and then a single Mac OS Extended (Journaled) volume is created. This will allow the erase, partition, deployment, and the installation of the drive.

2. The Disk Utility also fails in another circumstance like partitioning and formatting various USB drives. This is displayed as an error

"Disk Erase failed with the error: POSIX reports: The operation couldn't be completed. Cannot allocate memory."


The problem is associated with specific USB chipsets and firmware versions and settings. The standard procedures of resetting devices and reset of SMC and PRAM fails.


The initial formatting of the drive can be done with terminal commands, to be made accessible by Disk Utility again. The Disk identifier for the USB drive needs to be found out. The ‘diskutil’ or the ‘fdisk’ commands can be used. After these commands are used, we can retry using the Disk Utility.

3. A third error can be with an external hard drive attached with the Mac system.

"Disk Erase failed"


"Disk Erase failed with the error: unable to write to the last block of the device"

An attempt to partition the disk also produces the same error.


Any attempt to erase fails because the drive cannot be repartitioned.


The first option is to get the drive completely zeroed and then partitioning is possible and the GUID table can be set afterwards. The zero fixes the drive and then formatting is allowed. Formatting problem, if happening on a bad sector, still continues. The bad sectors are to be mapped and then the formatting is done.

4. A fourth reason is erasing an NWFS partition (Windows OS). The Boot Camp Assistant and the Disk Utility fails.


The file system was not recognised


Instead of erasing the partition, the partition tab in the Disk Utility can be used.

5. Another problem is

“disc erase failed with error invalid request”


This happens to an external hard drive to be used for both Mac as well as the PC, so that data can be backed up from the PC.


A partition scheme has to be picked in the partition tab. The GUID partition has to be chosen, instead of the “Master Boot Record”.

However, there are software like Mac Drive Partition Manager which can format, resize or delete existing partitions. This allows easy and quick formatting of partitions, when the entire data needs to be wiped for the removal of unwanted virus and malicious programs from the drive.