

The causes of this error are several, starting with the abrupt shutdown and inconsistent file system. What causes the 'Couldn't Unmount Volume' error? Usually, the error 'Couldn't Unmount Disk' shows up when you try to format, partition, verify or repair internal disk by Disk Utility on Mac system.Unmounting a disk means disconnecting a disk drive or optical disk from the computer.

To know about the solutions, first, let's understand about unmounting and disk utility, and the causes of the disk error. Part 1- What does "couldn't unmount disk" mean?

If Disk Utility reports that the disk appears to be OK or has been repaired, you’ve finished. In the sidebar, select the storage device, then click the First Aid button. Repeat steps 6 to 8 for each container on the storage device. In the sidebar, select a container, then click the First Aid button. Repeat steps 2 to 4 for each volume on the storage device. When the First Aid process is complete, click Done. In the First Aid dialogue, click Run, then follow the onscreen instructions. In the sidebar, select a volume, then click the First Aid button. If you check your startup volume (Macintosh HD), make sure you also check your data volume (Macintosh HD - Data). Next, select Disk Utility in the macOS Recovery window, then click Continue. See the Apple Support article About macOS Recovery on Intel-based Mac computers. Note: If you’re checking your startup disk or startup volume, restart your computer in macOS Recovery.
