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

On checking my Mac this morning, I found a dialog box I'd not seen before. It said that Disk Utility could not repair the disk, that I could read and copy the files on the disk but could not write to the disk - and something else that I forget but that was not new.

Then I came here to check on my query and found your reply, for which I thank you.

Here is the screen shot of Disk Utility, as I found it:

So I tried running Repair Disk again, hoping to get that same new dialog box. This, however is what I got:

I checked Finder, and sure enough, that Toshiba HD (my "original" Time Machine drive) is shown - but not the WD drive (newly purchased for this purpose). I tried to mount the WD drive, but I got this:

So I tried to repair the WD and got this:

Because I now seem to have access to the files on the Toshiba HD again but not to those on the WD, I tried to Erase the WD and got this:

I tried to Eject the WD from Disk Utility, with this result:

So I now can do nothing with the new WD drive. Is there a way out of this?

As for the Toshiba drive, I did in fact copy a test file successfully from the Toshiba drive to my Desktop. And I was able to Eject the drive in the Finder. So I might be able to use it as a kind of read-only, manual backup of my Mac's startup drive from the day I got it until last week - if the Finder sees it next time I connect it.

Thank you.

Yes. It came formatted for Windows. I used Disk Utility Erase and selected Mac OS Extended (Journaled).

I notice that when I select the partition in Disk Utility, it says (at the bottom), "Format : Mac OS Extended" - not "(Journaled)". Is the difference meaningful?

Disk Utility won't reformat. I choose Erase and tell it what to do, and I get "Disk Erase failed with the error: Couldn’t unmount disk."

So strange. I bought two of these drives, identical except for color, formatted them identically (I thought), used blue one as a new Time Machine drive and the black one to try to recover the data on the troublesome Toshiba drive.

The blue one says it's formatted as Mac OS Extended (Journaled), and it seems to work fine - when I have Time Machine turned on. I've kept it off the last two days while struggling with these other two drives. I've also kept my SuperDuper! backup disconnected.

I tried again to Repair, but I get

Error: A disk with a mount point is required.

Erase says "Unmounting disk" and shows a full progress bar, but then comes a dialog box:

Disk Erase failed with the error:

Couldn’t unmount disk.

Is there another way to reformat a disk?

Actually, I did that earlier and copied the output into my notes:

/dev/disk0

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *500.3 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_CoreStorage 499.4 GB disk0s2

3: Apple_Boot Recovery HD 650.0 MB disk0s3

/dev/disk1

#: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS Macintosh HD *499.0 GB disk1

Logical Volume on disk0s2

AFF78F9B-F150-4121-BFE7-077696BD0BA9

Unencrypted

/dev/disk2

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *2.0 TB disk2

1: EFI EFI 209.7 MB disk2s1

2: Apple_HFS Time Machine RMBP 2.0 TB disk2s2

Also, I tried

sudo diskutil unmountDisk force /dev/disk2

Result:

Forced unmount of disk2 failed: at least one volume could not be unmounted

And then:

hdiutil detach /dev/disk2

Result:

hdiutil: detach: timeout for DiskArbitration expired

Both of these because I read that folks had tried them and they worked - and because it didn't seem that I could make disk2 much worse.

Thank you for that. I've read that sudo is risky, but I did not know that I wouldn't need it in that case.

To make things more puzzling, at least for me: I connected the blue WD disk and ran diskutil list. The only difference from the one above is that there is now a disk3, and it's identical to the offending disk2, except for the names I gave them when I formatted them:

/dev/disk0

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *500.3 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_CoreStorage 499.4 GB disk0s2

3: Apple_Boot Recovery HD 650.0 MB disk0s3

/dev/disk1

#: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS Macintosh HD *499.0 GB disk1

Logical Volume on disk0s2

AFF78F9B-F150-4121-BFE7-077696BD0BA9

Unencrypted

/dev/disk2

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *2.0 TB disk2

1: EFI EFI 209.7 MB disk2s1

2: Apple_HFS Time Machine RMBP 2.0 TB disk2s2

/dev/disk3

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *2.0 TB disk3

1: EFI EFI 209.7 MB disk3s1

2: Apple_HFS 2 TB TM blue 2.0 TB disk3s2

Is there more to do? I must go out right now: life calls, and I have errands to run while the places are open.

macjack wrote:

Unless you partitioned a disk with more than one volume, that accounts for all your connected disks. I'm kind of at a loss for what the problem may be.

Just realized that I did not really respond to this.

No, I did not partition a disk with more than one volume. While trying to get the HD (the black WD, I think) to mount, I tried to use Partition to reformat the one partition. Disk Utility gave up in pretty much the same way.

macjack wrote:

I see 3 volumes all with proper mount points.

For my edification, what tells you that?

Thank you for your efforts and information.