Use Disk Utility
Disk Utility is used to manage and format devices, verify devices integrity, and erase volumes. It’s useful during macOS reinstalls or when resolving storage-device-related issues. Be cautious when erasing devices to avoid deleting critical partitions or user data unintentionally. Use macOS Disk Utility to perform and guide users through various storage-device-related tasks, especially when troubleshooting storage or system issues.
Here are the most common uses:
Running First Aid: Diagnose and repair storage or volume errors and when a user reports slow performance, unexpected shutdowns, or corrupted files.
Formatting and erasing drives: Prepare external storage devices for use, and change file systems depending on the use case.
Partitioning or managing volumes: Add or remove partitions or Apple File System (APFS) volumes, and set up dual startup environments or organizing data.
Mounting or unmounting drives: Help users when external storage devices don’t show up in the Finder, and manually mount volumes or diagnose mounting issues.
Creating or restoring disk images: Create backups or secure storage containers with encrypted .dmg files, or restore a drive from a saved disk image.
Checking drive information: Verify format, capacity, available space, and status, and determine whether a storage devices is failing or incompatible.
Working in recoveryOS: Access Disk Utility through macOS Recovery to repair or erase startup disks.
If an external volume won’t mount
What to look for:
After attaching an external volume, nothing appears on the desktop.
Disk Utility shows the drive as not mounted.
An error message shows that the disk couldn’t mount.
Steps to take:
Check the physical connection.
Remove and reconnect the drive.
Try a different USB port.
Use a different cable if possible.
Try a different power source if the drive needs external power.
Restart the Mac. A restart can reset the USB controller and disk mounting logic.
Open Disk Utility. Go to Applications > Utilities > Disk Utility. Look for the drive in the left sidebar. If it’s there but grayed out, select it and click Mount. If the Mount button fails, continue to the next step.
Use First Aid in Disk Utility:
In Disk Utility, select the external drive (start with the parent or volume at the top).
Click First Aid, then click Run.
Wait for it to check and repair disk errors.
Try mounting again after repair.
Check whether a device management service is managing the mount policy of external storage:
Check with your device management administrator.
Go to Apple menu > System Settings > General > Device Management, and look for a configuration profile that manages Media Restrictions.
If a drive shows read-only errors
What to look for:
When trying to copy, move, or modify files on the storage device, alerts like the following might appear:
“The operation can’t be completed because the disk is read-only.”
“You don’t have permission to write to this folder.”
“This item can’t be moved because [device name] is read only.”
Steps to take:
Corrupted file systems can force macOS to mount storage devices as read-only. Check for disk errors:
Open Disk Utility from macOS Recovery.
Select the storage device.
Click First Aid > Run. If it finds and repairs issues, the device may remount as read/write. If the repair fails, proceed with data recovery or reformatting.