In this case, you should be able to fix the issue by initiating a repair disk operation via the Recovery Menu. Corrupted OS files – Under certain circumstances, you can expect to see this issue occurring in situations where you’re actually dealing with some type of system file corruption that is affecting the reformatting operation.In this case, you need to flip the Locked/Unlocked switch in order to fix the issue. SD Card uses is ‘Locked’ – Keep in mind that if you’re using an older SD card (especially from SanDisk) when encountering this issue, it’s possible that the operation fails because the external device is locked, so new info cannot be written on it.If you find yourself in this particular scenario, you should be able to fix the issue by creating a new equivalent via Disk utility or directly via the Terminal app. Missing / Corrupted Partition map – One of the most common scenarios that might cause this problem is an instance in which the required partition map is either missing or severely corrupted.

Disk utility could not unmount disk