Q: Using Disk Utility to Erase Disk-won't unmount

Thank you so much! I grep'd fsck and kill the process, and my drive came back online and I was able to reinstall OS X. You Rule!!!

Thank you, desbrisay !!!!!! By grep'ing fsck and killing the process, my Macbook Pro's hard drive came back online, allowing me to Format the drive and reinstall OS X.

Great Advice!!!!!!!!

Thanks to Dana for the pkill tip and to everyone else for their suggestions here. I couldn't find fsck_hfs in the Activity Monitor. Booting into recovery mode wouldn't work for me either.

The process that sorted it for me was:

- Disable spotlight indexing in terminal using  sudo mdutil -a -i off

- Stop fsck_hfs process in terminal using  sudo pkill fsck_hfs

- Immediately open Disk Utility and Erase the volume

- Re-enable spotlight indexing in terminal using   sudo mdutil -a -i on

Cheers,

Chris

I am having similar issues and it's really frustrating, restarted command R, erase disk failed, restarted went to disk utility

tried to erase the disk from there but it's all greyed out.  Repaired disk permissions, disabled spotline via terminal, still greyed

out.  I used several commands listed on here in terminal

diskutil eraseDisk JHFS+ My\ HD disk0

diskutil eraseDisk JHFS+ UntitledUFS disk2 

But it won't unmount the disk, I'm using Yosemite and no matter what I do it won't let me erase

the disk on here, help?

Thank you for your suggestion! So far, it appears to have addressed the issue on my mid-2010 MBP (running 10.10.2), and my (USB) WD 500G EHD Time Machine back-up is once again visible/available in the Finder (and in the process of backing-up ~200G). The only real downside of doing this AFAIC was losing all of the previous back-ups on the EHD, but it's worth it (to me) if this proves to be a permanent fix.

FWIW, A Potential Source of the Issue:
While the EHD was engaged in its Time Machine back-up yesterday, I was simultaneously backing-up some of my files onto multiple thumb drives. This reinforces my suspicion that there exists some type of "simultaneous, multiple back-ups" conflict with Time Machine. On multiple occasions I've experienced similar issues with simultaneous Time Machine and thumb drive back-ups, and, except for this time, the disastrous result reported by Disk Utility was always the same: a damaged HDD (_not_ the EHD) that could only be repaired by erasing it. (As I recall, the Disk Utility error citation--paraphrased--indicated that the # of files were not identical as they should have been.). Based on my research in other Apple forums, Time Machine has been notorious for causing all kinds of strange problems. Has anyone else been able to pinpoint what may have been done to cause the EHD damage/corruption that caused it to malfunction and its Finder icon to vanish? I'm keeping my fingers crossed that all will still be well once the back-up is complete and I re-boot, and I'll report back if I experience any issues, so, hopefully, you won't hear from me again on this topic. ;-)

Nothing worked in my case.  No matter what, the disk would not unmount or reformat.  What did work was bootcamp.  Since my Windows volume was on another drive, I was still able to mount and reformat the drive as NTFS.  After the bootcamp reformat, I was able to restart and format the disk to Mac OS from the recovery menu. 

This was the technique that solved it for me. The other solutions above might have worked for me if I knew anything at all about Unix code and Terminal, but for a novice the easy way was:  sudo diskutil unmountDisk force /dev/disk<number>    Thank you eplt@odw

Thank you Coluch. So simple and it worked!

This was perfect for me.  After the 10.10.3 update, my backup stopped working.  Time Machine couldn't access the hard drive.  I could not get it to unmount so I could just reformat and start Time Machine fresh.  This solution finally resolved and allowed me to unmount and erase the drive.  Thanks

Thanks for your help with this. It worked. I was able to at least erase my disk. Much appreciated.

Great, it helped!!!

Unfortunately absolutely none of the solutions discussed in the various responses have worked for me with a WD My Book Studio which worked fine with my old MacBook Pro but never with my mid 2014 MacBook Pro running 10.10.4. I also tried WD so-called solutions and apps, none of which worked and in fact the installation of their apps seemed to go OK but these just disappeared and could not be found.

I did not try the bootcamp advice since I have no idea what that is.

Really fed up. Will stick to LaCie drives from now on!

This helped me, I was searching around for ages trying to find the answer.   I could tell the disk was doing 'something', turn out to be a fsck.

Made we wonder if there was a way to understand what processes were working with a particular device, you can use something like:

sudo lsof | grep '/Volumes/Macintosh HD'

This shows the open files on the device which could indicate to a rogue process?

None of the above suggestions worked for me. For instance,

sudo diskutil erasedisk JHFS+ Untitled /dev/disk4

Password:

Started erase on disk4

Unmounting disk

Error: -69888: Couldn't unmount disk

This is a LaCie XtremKey USB 3.0 that does not mount and cannot be formatted after the Mac Finder showed "Disk not ejected properly” (yet nobody ejected if, of course; it did it spontaneously) while saving a file to the pendrive. Any idea? Thanks.

wow this worked!!!