I am willing to dual-boot my MacBook Pro (13-inch, Early 2011) macOS Sierra to install Ubuntu.

I am having some trouble with my USB flash drive when I try to erase it using Disk Utility. It says:

Erase process has failed. Click Done to continue.

Unmounting disk
Couldn't open device.
Operation failed...

Erase process has failed. Click Done to continue. Couldn't open device.

  • For me the erase button was greyed out, so I View -> View All Devices did the trick (Source).
    – lony
    May 5, 2020 at 17:46

5 Answers 5


Open Terminal, and type

$ diskutil list
/dev/disk4 (external, physical):
0: FDisk_partition_scheme *8000.1 GB disk4
1: 0xEF 12.3 MB disk4s2

It cannot display the USB. To erase the USB completely, run:

$ diskutil eraseDisk free EMPTY /dev/disk4
$ diskutil list
/dev/disk4 (external, physical):
0: GUID_partition_scheme *8.1 GB disk4
1: EFI EFI 12.3 MB disk4s1

The first thing I would try is restarting your MacBook Pro in Safe Mode and trying to erase the USB flash drive then.

To start in Safe Mode:

  1. Fully shut down your Mac
  2. Restart your Mac
  3. Immediately press the Shift key and keep it down
  4. Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
  5. After booting up, insert your USB Flash Drive
  6. Launch Disk Utility and try erasing the USB Flash Drive
  7. To exit Safe Mode just restart your Mac as normal

If this does't work, take a note of any error messages and report back.

  • 2
    did not work for me, diskutil eraseDisk JHFS+ Untitled /dev/disk2 Error: -69877: Couldn't open device
    – user125388
    Feb 17, 2019 at 1:29
  • Getting error in disk utility, Couldn't Open device
    – Stivan
    May 23, 2019 at 14:19
  • Didn't work for me. I tossed the flash drive.
    – Ben Creasy
    Sep 22, 2019 at 4:50

I was getting this same error and discovered that the MicroSD Adapter I was using had a read-only switch enabled. Toggling the read-only switch off resolved the problem.

  • Doesn't really apply here since the Cruzer Edge has no read lock, but yeah, also with full sized SDs there's the same lock.
    – JMY1000
    Jan 14, 2018 at 22:06

I concur try rebooting.

I got around the issue myself trying to format an old PATA 80 GB drive to work with a truly old 2004 G4 Macmini: I had to try a few times but from memory what worked was to format the partition exFAT temporarily, then "erase" the whole disk choosing a new partition scheme from: - Apple (powerpc) - GUID (intel) - Master Boot Record (Windows)

The PowerPC macs use "Apple Partition Scheme". Perhaps ejecting or unmounting first helps.


Create a partition on the disk first

$ diskutil partitionDisk /dev/disk2 MBR FAT32 UNTITLED 100%

IMPORTANT: Replace disk2 with your appropriate disk number. You can find that by diskutil list command

Now you can proceed with erase.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .