Fix Guide: Tahoe Disk Utility 'Couldn't unmount disk' Error

Symptom: When trying to erase, partition, or Repair a disk in macOS Tahoe (26.x) Disk Utility, you get the error: "Couldn't unmount disk. : (-69888)".

Why this happens

This usually occurs because a background process (like Spotlight, Time Machine, or a Terminal window) is actively using the APFS container or a sub-volume. Tahoe's "Live Mount" feature sometimes fails to release the lock.

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Fix 1: Force Unmount via Terminal (Free)

Terminal is more powerful than the Disk Utility GUI.

1. Find Disk Identifier

• In Disk Utility, select the disk and look for the ID (e.g., disk4s1).

1. Run Unmount Command

• Open Terminal and type: diskutil unmountDisk force /dev/diskX (Replace X with your ID).

Fix 2: Quit Conflictive Apps (Free)

Ensure no system tools are "hooked" into the drive.

1. Stop Spotlight

• Go to System Settings > Siri & Spotlight > Spotlight Privacy.
• Drag the drive you want to unmount into the list.

1. Turn off Time Machine

• If the drive is a backup disk, turn off Time Machine in System Settings before trying again.

Fix 3: Use Recovery Mode

When the disk is the startup volume or heavily locked.

1. Enter Recovery

• Restart and hold Power (Apple Silicon) or Cmd+R (Intel).

1. Run Disk Utility

• Since the main OS isn't running, the drive will be easier to unmount and repair.

Recommended Tool: DaisyDisk or Disk Drill

When Disk Utility fails to unmount a drive, DaisyDisk is excellent for visualizing exactly which hidden file is "using" the disk. Alternatively, Disk Drill can help if the unmount failure is due to low-level partition corruption that needs a deep scan and repair.

[Fix Disk Errors with DaisyDisk]