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

I had this problem with a 2TB external Seagate disk. I thought the disk was just plain dead.

DiskUility could not repair it, which was fine. It's just a backup disk. But I wanted to repartition it.

BUT, it would not mount, and it would not unmount, by any means, force or not. It was in a kind of sad HD limbo.

Turning off Spotlight did not help either. (BTW, I think it was Spotlight (in conjunction with TimeMachine) which caused the problem to begin with.)

But what did help, after turning off Spotlight, was to kill a crazy fsck_hfs process that was hitting the disk in question so that DiskUtility could not unmount it.

ps -ax | grep fsck

sudo kill <process number>

where "<process number>" is the actual process number returned by ps. I found that solution here. And it works! read that whole thread.

Alternatively, you could probably also kill that crazy fsck_hfs process in Activity Monitor, where you will see it eating up your CPU.

After you kill it, then launch DiskUtility and it will now successfully unmount the disk and let you repartition it (or whatever).

BTW, I think the preferred way to turn Spotlight On/Off is a little more graceful than using the mdutil command.

Try this instead, to turn it off:

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

and then just turn it back on again when done:

sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

In summary, there are steps in this fix:

1.) turn off Spotlight;

2.) kill the fsck_hfs process;

3.) launch DiskUtility and format the disk;

4.) turn ON spotlight;

5.) restart is probably a good idea.

Hi, I've just solved the same problem successful 100%.

Step 1: Right after turn macbook on, press "Option-Command-P-R" to Reset NVRAM (See more Startup key combinations for Mac - Apple Support)

Step 2: Fix with USB Boot Drive (find how to create USB Boot https://www.youtube.com/watch?v=W7bPfNREVMU)

Step 3: Select Hardrive you want to fix + select First aid tab + "Verify" and "Repair"

Step 4: Press "Command + R" to reinstall normally.

Be successful!

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 🙂

There must be something on the drive that has a connection to the internal drive, I think. Try booting from the Recovery HD:

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the arrow button below the icon.

Select Disk Utility from the list and click on the Continue button. See if this lets you erase the drive.

To list all the drives and their accompanying partitions from the Terminal, type the following command:

diskutil list

This will give you a listing out the drives, their volume names, the size of the drive and partitions, their partition types, and their identifier location. Paste the disk drives listing into a reply to this message.

The external HD is for my Time Machine **. Does that make a difference? No difference.

Unable to use Recovery HD: Probably the Recovery HD is damaged.

Open the Terminal, and type the following command:

diskutil list

and paste the output into a reply to this message.

Here's a suggestion you might try:

It appeared that the external drive would not unmount because some activity was using it. fsck_hfs process was working away as shown by activity monitor. It seems that Spotlight was struggling with a the failed drive and would not let it go so the unmount and erase could not occur.

This was solved by disabling Spotlight (terminal: sudo mdutil -a -i off) temporarily. After disabling Spotlight, Disk Utility performed its job and my corrupted drive was restored to use.

(Spotlight is turned back on with: sudo mdutil -a -i on)

This is from a 2012 discussion re: this problem. It helped some users, so give it a go.

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

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...