Disk won't format

Purchased a 3Tb MY BOOK External Drive today. Following site procedures for reformatting to Mac OS Extended (Journaled) and erasing the disk using Mac disk utility, I received the message :slight_smile: “Unmounting disk
MediaKit reports not enough space on device for requested operation.
Operation failed…”

Don’t understand the space issue and since the disk is unmounted II am unable to do anything with it. Ubable to find a soluton. Need help

I am running macOS Sierra version 10.12.4 on a MacBook Pro.


What option are you using to format the drive? If you are using partition, please try using the erase option.

See if this helps

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I wasn’t using partition. I was erasing when I received the message about space. I subsequently ran disk first aid and got the message "Problems were found with the partition map which might prevent booting”. In fact the computer doesn’t recognize the disk at all and I cannot access it since it will not mount.

What’s up THOMAS,
I just bought an external disk and since I own a mac I also came to this site to format the disk so I’m able to use it in my mac. Using the Disk Utility I erase the disk as the instructions indicate, didn’t work, I got a message:
“Unmounting disk
MediaKit reports not enough space on device for requested operation.
Operation failed…”

Long story short, I figured out how to fix it. You will need a PC with windows.

  1. From the PC download the ‘WD Quick Formatter For Windows’ (maybe you can actually download the mac version on your mac, but in my case I have Sierra and the software is not supported, that is why I had to use a PC)
    Download the software from this WD article:

  2. Unzip the file on your PC and run the program.
    (Make sure you unzip the content on the indicated folder: C:\Users\xxxxx\AppData\Local\Temp\wz1be1\WDSmartWare )

  3. Run the ‘WD Quick Formatter.exe’ This will format again your external disk and will set it to PC format.

  4. Eject and reconnect to Mac. Open ‘Disk Utility’

  5. Clicking on the Main Disk (not the secondary) click on erase an put as format: MS-DOS(FAT), and put the name of the disk. Click Erase.
    (you can find detailed instructions from here: http://osxdaily.com/2012/04/22/format-drive-mac-pc-compatible/ )

  6. If everything went OK, the format should be completed, and you will be able to use the External disk in both PC and iOS.

**** If it doesn’t work and you need to format again you can always go back to the PC and use the ‘WD Quick Formatter’ and it will reset it as when you took it out from the box.

I hopes this help you, and anyone else with the same issue.

Ivan out.

I am having the same problem. Do you mean that it’s not possible to format this drive for a mac? only in exfat? That’s not really fully compatible at all.

When you use Disk Utility, use the View icon in the toolbar, it’s in the upper left-hand corner of the window. Change it to Show All Devices. Select the non-indented item, usually, it will have the name of the device (Western Digital or WD followed by the model). When you select Erase, you should get three fields to fill in:

If you don’t see a field for “Scheme” you are selecting a partition and NOT the parent drive. This article on Seagate drives has very specific instructions with images and TWICE they point out that you have to select the device. The problem is that by default Disk Utility does NOT show you the device names. Kind of dumb decision by Apple IMO.


Thank you for the solution.

Thank you !