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

Try again. 😀

I am running Mavericks 10.9.2 and am having the same problem, but am unable to fix it with the help suggested by Kappy and Dr.Mac. Specifically, I have a WD Passport external hard drive (formatted FAT) with a single partition that doesn't show up in Finder when I plug in the HD. I can find the disk in Disk Utility, but cannot repair it, eject it, or unmount it when I try to erase and/or re-partition the disk.

Things I have tried: unplugging the HD and plugging back in, restarting my computer, restarting and holding command+r to enter disk utility, entering Terminal and typing "diskutil unmountDisk force disk1", disabling Spotlight in Terminal ("sudo mdutil -a -i off") then trying to force disk unmounting (above command and "sudo diskutil unmountDisk force disk1")

....and still the disk will not unmount! What should I do?

BTW, here is the response in Terminal after the command diskutil list:

/dev/disk0

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *500.1 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_HFS Macintosh HD 499.2 GB disk0s2

3: Apple_Boot Recovery HD 650.0 MB disk0s3

/dev/disk1

#: TYPE NAME SIZE IDENTIFIER

0: FDisk_partition_scheme *1.0 TB disk1

1: DOS_FAT_32 BEAVER 1.0 TB disk1s1

ragarose, did you ever get an answer to your problem?

I am having the same problem as yours, that is, slightly different than the original poster.

I had the same error message when trying to reformat and reinstall Mavericks, but was unable to follow all the steps outlined, but did restart the computer and simply turned off Time Machine, then again restarted and was able to erase the hard drive without the error.

I'm having the same problem. My WD external disk won't unmount. I tried the Terminal command you suggested and got this response:

Forced unmount of disk1 failed: at least one volume could not be unmounted

Any other options?

Just had the same issue (yesterday) with USB drive (USB3.0) would not un-mount that had a (2nd) Time Machine backup on it.

I actually thought the disk died, because I just tuned off the power to the disk (I know it's not a good idea to power down without un-mounting). When I powered the disk back up the drive would not mount even after a restart. I took the disk out of the Voyager S3 and put it into a external enclosure and bam the disk was working again. I repaired the disk (using DU Repair Disk, no errors) and put it back into the Voyager S3 and the disk is fine now.

So I have NO idea what happen other than a possible Maverick bug of some sort.

Had the exact same problem, disk would not unmount no matter what I did. I solved the problem by taking the external disk to a windows box and cleaning it using the diskpart command line tool and the clean command. Then plugged it back into my mac and erased it. It All works now...

Thank you so much Kappy!!! That was huge...it worked, and within seconds at that! My external hard drive is back! And I didn't even have to restore it!

A million thanks...

Thanks Kappy. This solved my problem ..:)

Ok, so same problem - found this thread, etc.

Tried 2 different Imac's, Macbook and nothing. It won't unmount. I believe it has to do with the fact that TimeMachine backups were on this external WD 1TB drive.

So, used an ASUS Laptop with WIN 8.1 and ran the following:

http://wdc.custhelp.com/app/answers/detail/a_id/3865/~/how-to-partition-and-form at-a-wd-drive-on-windows-(8,-7,-vista,-xp)-and-mac-osx

I was able to delete the drive and then reformat with MAC.

Hope this helps others.

I had this problem on 10.10.1 with an external Firewire drive when trying to erase it with DiskUtility. No change after disabling Spotlight. I had my GoPro Hero3 also mounted as another drive. I ejected it and then DiskUtility was able to erase the Firewire drive.

Hope this is helpful to someone else who ends up here.

This was fabulous - helped me reformat an external hard drive to mac - wasn't letting me format until I turned off spotlight. it was trying to index the new hard drive. THANK YOU!

Hi Kappy, similar problem here

I used an external hard drive to install Yosemite (creating a single partition) and boot my computer from there, as I had a problem with my internal hard drive.

Now everything is working fine with my computer and I wanted to format the external hard drive. But it happens that when I try to erase the partition (Mac OS Extended or create new ones with disk utility I get the message "couldn't open disk" or "couldn't unmount disk". I've tried to verify/repair disk and, after a couple error messages ("This disk needs to be repaired" and "Disk utility can't repair disk, erase it") I'm in the starting point.

The "stop spotlight" thing didn't change anything, neither the "sudo diskutil unmountDisk force disk1" option

If I enter "diskutil list" in Terminal, what I get in response is:

/dev/disk1

/dev/disk2

Don't really know what to try now.