Error: -69888: Couldn't unmount disk

Hey guys

I'm trying to help a friend formatting a hard disk.

In recovery-mode he tried "diskutil secureErase 0 /dev/disk0" resuming "Error: -69888: Couldn't unmount disk"
Obviously there is some process on disk0s1, disk0s2 or disk0s3 blocking the unmount command and the following efforts to force unmounting or formatting (diskutil eraseDisk JHFS+ MacintoshHD disk0): Error: -69888: Couldn't unmount disk!

Who can help me and tell me what to do in Mac Terminal?

Normally, I used the "Disk Utility" app in macOS to format drives:

Naturally, you will need to unmount the drive(s) before you attempt to format them, etc.

You cannot unmount the drive when that drive is the boot drive, as I recall, even in recovery mode, if you booted off the same disk.

In your case, you cannot run diskutil secureErase 0 /dev/disk0 because you are using disk0s3 for recovery.

You can only unmount the other partitions, disk0s1 and disk0s2 and format them, and I'm not sure about the EFI partition.

Hey Neo,

thanks for your key advice.
He did not tell me and I did not recognize that my friend was trying to format this HD while in use as boot drive.

Sorry, I must have been blind...

Thanks again and bye for now.

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