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How to Fix "Couldn't Unmount Disk" Error


Have you ever met “couldn't unmount disk” error on your Mac? How to fix it without data loss?
I have found myself recently experiencing a couple of Macs which would not allow me to repair the directory or permissions in Disk Utility, or erase/partition the drive with an error such as: ‘Disc erase failed couldn’t unmount disc’ or “Disk Erase failed with the error: Couldn’t unmount disk.” why? This error can happen during partitioning, disk verification and repair, and even during formatting, and there’s usually little to no additional information provided as to how to resolve it, this is really frustrating.
However, it still can fail to fix disk issues accompanied with some error messages, such as “Couldn’t Unmount Disk” error. This error generally results from improper modification of the boot drive, such as partitioning, disk verification or repairing or formatting. When this error pops up, not only will the Disk Utility halt suddenly, but also the disk will suffer as well, such as damaged data on this disk. Therefore, it is imperative to resolve this error as soon as possible. Here we will share you 3 solutions.

Methods to fix “couldn’t unmount disk”

1. Resolve with USB Boot Drive

If this error occurs on the disk which is the boot drive, the simplest solution is to boot the Mac system from another drive and then run Disk Utility on the disk. By this means, you can make use of USB boot drive.
       1. First off, insert the USB boot drive to Mac and reboot.
       2. Then, during booting, you should hold down the “Option” key button to select the attached boot drive.
       3. Next, in the boot menu, select “Disk Utility”.
       4. After that, go to “First Aid” and verify the disk.
       5. Lastly, just perform the original task on the original disk.

2. Fix with Recovery Partition

If the error message appears during “First Aid” or formatting a non-boot disk, the above means will not make effects surely. At this point, you can try solving it with Recovery Partition.
        1. For a start, reboot your Mac.
        2. Then, when booting, hold down the “Option” key to select Recovery Partition.
        3. Next, access Disk Utility from the boot menu.
        4. Lastly, select “First Aid” to re-verify the disk or choose “Erase” to format the disk at your will.

3. Solve with Command Line

Moreover, you can also attempt to eradicate this error, forcing unmounting disk by command line. Follow the steps below:
        1. To begin with, you have to access Terminal application.
        2. Then, input the “diskutil list” line, which will list out all the disks on the Mac.
        3. At this time, you ought to find out the disk that you want to force unmount.
        4. Lastly, you should type “sudo diskutil unmountDisk force /dev/diskx” line. Replace the “/dev/diskx” with the correct disk number, such as “/dev/disk1”.
If you found your drive in unmounted condition on Mac and urgently need to recover the data then recover them as below steps.

Steps to recover data from a unmounted drive on Mac

Download and Install Data Recovery for Mac software

I recommend you to use Bitwar Data Recovery for Mac to recover your lost data from your drive. It is a powerful, fast, and all-around data recovery software program for Mac, which can be used to recover multiple types of files from different locations including the Mac platform and even external devices.
Download and install Bitwar Data Recovery for Mac by click download icon below for your Mac. It is compatible with OS X 10.11 and later.
Tips: For latest mac OS 10.13 (High Sierra) system requirements, Mac users are not allowed an access to system disk from apps. If you want to recover data from the system disk, please disable the system feature "System Integrity Protection (SIP)" , see the guide on How to Disable System Integrity Protection (SIP) in macOS High Sierra (macOS 10.13)

 Bitwar Data Recovery for Windows download Bitwar Data Recovery for Mac download

Step 1: Select the unmount drive

Launch the Bitwar Data Recovery program and select the unmount drive on your mac, then click "Next".
couldn't unmount disk

Step 2: Select the Scan Mode

Select “Quick Scan” and click “next”. If quick scan could not scanned lost files, try deep scan
● Quick Scan: Recover files from accidental deletion, such as command + deleted, emptied the Trash or deleted by other software, etc. The files can be retrieved completely in condition of the files or directory structures haven’t been overwritten by other data.
● Formatted Recovery: Recover formatted files after performed a quick format on a partition or hard drive.
● Deep scan: If previous scan could not worked to find lost files, try deep scan this time. Deep scan will search every sector of hard drive to find all existing files. But the scanning process will spend more time than quick scan and will not have the original file name in the scanning results.
couldn't unmount disk

Step 3: Select the file type

Select the types of files you want to restore on this step. If you have no idea about what types to recover, choose “Select all types”, because you can recover all types in Bitwar data recovery for mac.
couldn't unmount disk

Step 4: Preview, recover and save files

After scanning, you can filter the lost files by name or Type. Then preview them to make a selective recovery. After the decision, select the data you want and press “recover” to save them to your Mac computer.
couldn't unmount disk
After the four steps above, the data on the unmount device on Mac have been recovered.
Bitwar Data Recovery for Mac supports the recovery of files from an external hard drive on mac OS High Sierra and other mac OS and OS X variants. Further, it provides support for data recovery on APFS, HFS, HFS+, NTFS, FAT and ex-FAT external hard drives.

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