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I have two internal drives in my 27" iMac (10.8.5). An SSD system disk and a 1TB SATA drive.

The HDD has been acting up recently, very slow performance, bad behavior etc. Tech Tool Pro and Disk Utility don't seem to be able to fix the drive, so I decided to wipe the data by a reformat and writing zeros to the entire drive.

Everytime I try and format, it tells me that the drive cannot be unmounted. I have rebooted a few times, and spent days running everything from TechTool Pro on it and it won't go.

$ diskutil list disk2
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk2
   1:                        EFI EFI                     314.6 MB   disk2s1
   2:                  Apple_HFS Everest                 4.0 TB     disk2s2

$ diskutil repairVolume /dev/disk2s2
Started file system repair on disk2s2 Everest
Updating boot support partitions for the volume as required
Error: -69673: Unable to unmount volume for repair

$ diskutil eraseDisk JHFS+ Everest /dev/disk2
Started erase on disk2
Unmounting disk
Error: -69888: Couldn't unmount disk

Any thoughts on how to get it to unmount and reformatted? Is there a way in Terminal to do this instead?

share|improve this question
    
@bmike Since it's unclear, if it's really a duplicate of the linked question, i wouldn't add the diskutil list here. Actually I think the reasons are different: here probably a degraded hdd, there a wrong EFI size and therefore an unreadable or partly overwritten HFS+ volume. The symptoms (unable to unmount) and the solution (force unmount) are probably the same though. – klanomath Mar 25 '15 at 17:02
    
no most likely an early fusion drive – oemb1905 Oct 10 '15 at 3:49
up vote 26 down vote accepted

You can force unmount a drive by running the following Terminal command:

diskutil unmountDisk force /Volumes/VOLUMENAME

Replace VOLUMENAME with the name of a volume on the disk you are trying to unmount.

Following this, attempt to Erase/Partition the drive again in Disk Utility. Using the above command can interrupt file read/writes, which can cause file corruption. However, since you are erasing the drive anyway, this doesn't really matter in your situation.

share|improve this answer
    
Thanks! It is now writing zeros to the drive. – Kray Oct 18 '13 at 17:47
    
Thanks. Combining this with ^ ... worked perfectly. Apple - please make your GUI stronger. – oemb1905 Oct 4 '15 at 7:29
    
Didn't have a volume to unmount; unmounted the disk device instead (with N being 3 in my case): diskutil unmountDisk force /dev/diskN – Joel Purra Feb 10 at 19:35
    
I was able to unmount the disks using above solution. But now I am getting following error while trying to erase or paritition primary internal HD: "wiping volume data to prevent future accidental probing failed". Any help is greatly appreciated. – Ab'initio Jun 9 at 10:42
    
@Ab'initio This should be a separate question rather than a comment. Make sure to search before asking. – grgarside Jun 9 at 11:33

If you have a PC download transmac trial start in administrator mode plug your disk in via usb. Locate your drive and right click on it. If you have an image you would like to clone to it use the clone option. Otherwise use format and choose HFS+ plug the hard drive into your Mac again and it should format no problem.

Worked for me this morning. Took about 5-10 minutes.

share|improve this answer
    
Transmac worked for me also. Spent hours trying other solutions, but this worked straight away! – user128386 May 18 '15 at 23:30

Disable Spotlight for that Hard Drive (System Preferences)

https://discussions.apple.com/thread/1454712?start=0&tstart=0

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