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

I was having the exact same problem in Yosemite and I discovered a simple solution without any Recovery HD or terminal stuff. It was just a Time Machine issue.

In my case, I was trying to erase an external HD that was different from the last one I used with Time Machine. The easy solution:

1. Open Time Machine Preferences

2. Turn off Time Machine if it is on

3. Select the available external HD that you want to erase

4. Click "Use Disk"

5. If you get another dialog box, chose the "Replace. . ." option to get the previous disk out of the way

6. Now you should be able to erase your external HD

7. Reset Time Machine however you want.

I have the same problem but its my internal Hdd, I had my harddrive replaced so they pre-installed mavericks on it, but I want to revert back to mountain lion so I can use my interface and other musical hardware, but when I erased the hdd and did a clean install from a usb thumb drive it goes thru as if its installing but when it's done and I check the system preferences it keep saying 10.9.4 but it has the background of the mountain lion osx I"m stuck don't know what to do can someone please help, I've tried almost everything that was posted here!...

You'd think bluesimage would be able to come to the rescue of Ladysoul221, wouldn't you? Unfortunately, I don't have a simple solution for you. I recently updated from good old Snow Leopard (10.6.8) to Yosemite because more and more of my apps were becoming obsolete. It wasn't the easiest process and I had to add some RAM to two of my Macs. Now everything is cool and running smooth. Are there any updates available for your musical gear that would help you upgrade to the free Yosemite OS?

Hello there,

This is my very first post on Apple Support Communities. I formatted my disc yesterday. It had a Mac and Windows partition and was getting slow. So I decided to format the entire thing and mainly partition it to Windows because that about all I used between the two partitions. When I formatted it, my first move was to upgrade the Yosemite but I highly doubt that's the issue. I then updated everything and opened up Bootcamp Assistant to install Windows. I partitioned 351 GB to Windows and 149 to Yosemite. It worked but when it restarted to initiate the Windows 7 Ultimate installation, it said "Non-system disk. Press any key to reboot". Pressing any keys didn't work. So I had to force shut it down. I tried several times and had the same outcome so I resorted to reformatting the entire disc again. I booted into the Mac partition and removed the BOOTCAMP partition. Once I did, I proceeded into the Recovery sector where I was going reformat the physical disc itself called "Toshiba (whatever the model number is)". That however, gave me the error on the system's inability to unmount the disc. I could format "Macintosh HD" which was under the Toshiba but not the Toshiba itself. This is all very vexing because I mainly work on Windows. Any ideas on how to resolve this? 😟

Hi TheMarked, welcome to Apple communities. Unless your issue is the same or almost the same as an O.P. (original poster), you are best advised to post your own, new question which will gain more attention from people who can help, rather than tack it on to another discussion. Your issue has some particular unique aspects which might be best served on its own discussion. :-)

I had this same issue and whatever user said it was because there was an operation running was absolutely right! I didn't realise it, but I had opened one of the driver files that came pre-installed on the external hard drive, and even though I had closed the file, the driver was still running (the little disk drive icon was on my desktop). So I ejected that, along with the external hard disk itself. Then I plugged the drive back in and everything worked fine--it erased and formatted and I just finished a successful Time Machine backup.

Good luck! ;-)

I had the same issue. Just updated to 10.10.4 on my iMac everything was working fine when Time machine auto backed up then the iMac restarted. I resumed the back up manually when time machine prompted that it is unable to back up. Ran disk utility and told me to repair the back up thats when all the problem of unmountable partition occurred. Disk utility unmounted the partition to verify and repair then it was unable to repair the partition told me to back up everything and reformat the drive. Tried to erase the drive and Disk U prompted it is unable to erase the drive since one partition is unmountable. Switched to the iMac and MacBook Pro to erase the external HD to no avail. We have no windows computer at home. I used a Sony Bravia TV to reformat the corrupted WD MyPassport Slim HD and to my surprise it worked. I suppose volumes corrupted and unmounted by Mac OS can only be fixed by other OS?

THANK YOU SO MUCH, YOU SAVED MY MAC YOUR A LIFESAVER 🙂

I couldn't figure out what was causing so much grief on the 2TB external. SPOTLIGHT! It probably had not been mounted since before Mavericks. So it was just slogging and not unmounting. Killing spotlight definitely solved the problem in my case. Disk Utility started doing its thing after running that command.

I'm wondering if spotlight will start up on a reboot.

This definitely worked for me, now my external back-up is fresh! Thanks guys!

Thank you SO much. Just spent two full days looking for a solution... My Transcend JetDrive SD card wouldn't erase after I saved my Dropbox folder on it and Dropbox updated the software instead back to my HD instead of the SD...

But now I was able to erase everything on the SD with no problem from the OS Utilities window after restart.

Learned something valuable about my dearly beloved MacBook Air thanks to you!

I had the same problem trying to erase my Yosemite Time Machine backup drive (a Seagate 3TB external) after upgrading my MacBook Pro to El Capitan. Disk Utility wouldn't erase it because it couldn't unmount the drive. What worked for me was to add a partition from the Partition tab in Disk Utilit. That process took a good 5 minutes. I then was able to erase the drive, which reverted to a single partition. I erased it again to be on the safe side, although that probably was unnecessary. I have no idea why that worked, but I then was able to use the drive for my El Capitan Time Machine backup.

I just had this problem, while erasing the hard disk from my El Capitan install USB drive.

I checked this page and it suggested, correctly, that the drive was not unmounting properly.

Rather than using Terminal, there is a much simpler solution:

1. In Disk Utility Click the hard disk NAME (eg. Macintosh HD) on the left hand list - not the drive itself (eg. Toshiba1234 or Samsung1234).
2. Click on the Mount tab and Unmount the drive. The name on the left should go grey.
3. You should now be able to select the drive and erase it.

Worked for me!
Simon 🙂

I just came to the same solution! Thanks. Wish I had seen this about an hour ago!

don't partition the physical disk ie the upper name. erase the volume first ie the lower name then parition the physical disk.