denke Author

Level 1

69 points

Disk Erase failed with the error: Couldn’t unmount disk. What now?

First, Time Machine failed, with the infamous Read Only error.

I tried to Repair Disk with Disk Utility. Disk Utility could not complete the repair and said to copy the files I could and then reformat the drive. Trying to copy the files in Finder went so slowly that it would have taken weeks. After reading that it would not have worked anyway, I tried the next thing.
Using Restore in Disk Utility was suggested, so I tried it. That left me with two drives in the same condition. (One was brand new, purchased for this purpose.)
1. Neither drive is seen by Finder, even after restarting or shutting down.
2. Both drives are seen by Disk Utility, but the (single) partition on each is grayed out.
Disk Utility cannot:
1. Repair either disk.
2. Eject either partition.
3. Erase either disk.
4. Mount or unmount either partition. It says a drive with a mount point is necessary.
I restarted with Cmd-R:
1. When I tried Restore from Time Machine, no Time Machine drives were found.
2. When I selected Disk Utility, the result was the same as when I started up normally.

I have two questions:

What do I do now?
If I can gain access to either drive again, would the Time Machine backups (going back to 2013) useful/usable and thus worth saving?

Thanks for any help!

Posted on Apr 20, 2016 6:50 PM

21 replies

denke Author

Level 1

69 points

Apr 21, 2016 9:04 PM in response to macjack

macjack wrote:

They are all mounted at / or the root directory, which is the proper mount point.

So is it the disk name (e.g., /dev/disk0) that shows where it's mounted? Is it the first "/" or the second?

Am I understanding correctly: Disk Utility (or diskutil) shows a proper mount point but claims there is no mount point?

Where do I go from here? It seems to me it's a problem with Apple software (Disk Utility). Should I call Apple? Go to a Genius Bar? Upgrade to El Capitan?

macjack

Level 10

111,242 points

Apr 22, 2016 9:15 AM in response to denke

denke wrote:

So is it the disk name (e.g., /dev/disk0) that shows where it's mounted? Is it the first "/" or the second?

Am I understanding correctly: Disk Utility (or diskutil) shows a proper mount point but claims there is no mount point?

Where do I go from here? It seems to me it's a problem with Apple software (Disk Utility). Should I call Apple? Go to a Genius Bar? Upgrade to El Capitan?

It's the first / that stands for your root directory. Take it one by one to understand... In Terminal you can type cd / (Change directory to root) at the prompt then press Return, thentype ls (list root directory)and Return. This will show everything in the root directory, you will see Volumes in the list. Then you can type ls Volumes and Return to see your mounted drives.

You can try the Genius Bar, but it seems more an issue with the drives than with Disk Utility. Let us know.

denke Author

Level 1

69 points

Apr 22, 2016 12:31 PM in response to macjack

macjack wrote:

It's the first / that stands for your root directory. Take it one by one to understand... In Terminal you can type cd / (Change directory to root) at the prompt then press Return, thentype ls (list root directory)and Return. This will show everything in the root directory, you will see Volumes in the list. Then you can type ls Volumes and Return to see your mounted drives.

Thank you. (Typing cd / reminds me of the MS-DOS days.) And ls lists only the mounted volumes (e.g., those shown in Finder), while diskutil list yields all the drives shown by Disk Utility, including the unmounted volumes - correct?

macjack wrote:

You can try the Genius Bar, but it seems more an issue with the drives than with Disk Utility. Let us know.

But once I formatted the black WD drive, it worked fine: using Finder, I copied several Time Machine backup folders from the Toshiba “original” Time Machine drive to the black WD drive. It took so long that I started looking for alternatives, but otherwise everything seemed in order: no error messages, a test PDF document opened correctly, an Application opened okay.

Then I read that this Finder copying procedure would probably not produce what I wanted and that the appropriate method is to use Restore from Disk Utility. So I did that, using Disk Utility to Restore the Toshiba Time Machine backup disk to the black WD disk. Only then did the black WD drive start having the same “Read only” issues as the Toshiba drive. And I’ve yet to have a problem with the blue WD drive, which is the same model as the black one except for color.

It seems unlikely to me that the new WD drive would coincidentally have the same issues as the several-year-old Toshiba (OWC) drive only and immediately after restoring the Toshiba drive to the WD drive using Disk Utility. It looks as if Disk Utility copied the problem from the Toshiba drive to the WD drive, which I’d think means it is a software issue. What am I missing here?

macjack

Level 10

111,242 points

Apr 22, 2016 1:01 PM in response to denke

denke wrote:

Thank you. (Typing cd / reminds me of the MS-DOS days.) And ls lists only the mounted volumes (e.g., those shown in Finder), while diskutil list yields all the drives shown by Disk Utility, including the unmounted volumes - correct?

It seems unlikely to me that the new WD drive would coincidentally have the same issues as the several-year-old Toshiba (OWC) drive only and immediately after restoring the Toshiba drive to the WD drive using Disk Utility. It looks as if Disk Utility copied the problem from the Toshiba drive to the WD drive, which I’d think means it is a software issue. What am I missing here?

Use the disutil list command to check the mount point of the drives and their path. The cd / and ls command was only meant to show you how to drill down to different directories.

Yes, when you explain it that way, trying to copy TM files back and forth improperly could have caused the issue. Let us know what you find out at the Genius Bar.

denke Author

Level 1

69 points

Apr 24, 2016 4:30 PM in response to macjack

macjack wrote:

Let us know what you find out at the Genius Bar.

Unfortunately, all I found out at the Genius Bar is that they accepted my assessment that it was an Apple software problem but could not diagnose or resolve it in any way. It was beyond their capacity.

So I took the drive to Best Buy, where I'd bought it. They confirmed that the drive was malfunctioning, reformatted it at my request, and exchanged it for a new drive. I reformatted the new drive for Mac and am using it as a second Time Machine drive. (The Time Machine drive on my wife's iMac broke last year, so now I'm feeling that one Time Machine drive is not enough.)

macjack

Level 10

111,242 points

Apr 24, 2016 4:36 PM in response to denke

Thanks for the update. I'm glad to hear it's resolved. 🙂

You can never have enough backups!

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Disk Erase failed with the error: Couldn’t unmount disk. What now?