Disk Utility User Guide
Disk Utility can fix certain disk problems—for example, multiple apps quit unexpectedly, a file is corrupted, an external device doesn’t work properly, or your computer won’t start up. Disk Utility can’t detect or repair all problems that a disk may have.
If you run First Aid on a disk, Disk Utility checks the partition maps on the disk and performs some additional checks, and then checks each volume. If you run First Aid on a volume, Disk Utility verifies all the contents of that volume only.
In the Disk Utility app on your Mac, choose View > Show All Devices.
Note: If you’re checking your startup disk or startup volume, restart your computer in macOS Recovery, select Disk Utility in the macOS Utilities window, then click Continue. If you check your startup volume (Macintosh HD), make sure you also check your data volume (Macintosh HD - Data).
In the sidebar, select a disk or volume, then click the First Aid button .
If Disk Utility tells you the disk is about to fail, back up your data and replace the disk—you can’t repair it. Otherwise, continue to the next step.
Click Run, then click Continue.
If Disk Utility reports that the disk appears to be OK or has been repaired, you’re done. You can click Show Details to see more information about the repairs. Otherwise, you may need to do one of the following.
If Disk Utility reports “overlapped extent allocation” errors, two or more files occupy the same space on your disk, and at least one of them is likely to be corrupted. You need to check each file in the list of affected files. Most of the files in the list have aliases in a DamagedFiles folder at the top level of your disk.
If you can replace a file or re-create it, delete it.
If it contains information you need, open it and examine its data to make sure it hasn’t been corrupted.
If Disk Utility can’t repair your disk, or you receive a report that the First Aid process failed, try to repair the disk or partition again. If that doesn’t work, back up as much of your data as possible, reformat the disk, reinstall macOS, then restore your backed-up data.
Since 1998, DiskWarrior has won more awards than any other Mac disk utility. A Macworld Eddy, an Apple Design Award, a PC Mag Editors' Choice Award, a MacUser Magazine Editors' Choice Award and many more from around the world. Check out our Awards and Reviews. Developed just for Mac. No.1 disk formatting tool for Windows and Mac – Built-in Formatting tool. Both Windows and Mac OS offer built-in disk formatting tool. For Windows, it offers quick format feature when you right-click the hard drive on the computer. If you are using a Mac, you can run the built-in Disk Utility to quickly format a hard drive or external hard.
If your Mac has a Fusion Drive and you see a flashing question mark or alert, see the troubleshooting section of the Apple Support article About Fusion Drive, a storage option for some Mac computers.
If you continue to have problems with your disk or it can’t be repaired, it may be physically damaged and need to be replaced. For information about servicing your Mac, see Find out how to service or repair your Mac.
Disk Utility User Guide
You can use Disk Utility to create a disk image, which is a file that contains other files and folders.
Note: You can burn information to a CD or DVD using the Burn command in the Finder. See Burn CDs and DVDs.
Create a blank disk image for storage
You can create an empty disk image, add data to it, then use it to create disks, CDs, or DVDs.
In the Disk Utility app on your Mac, choose File > New Image > Blank Image.
Enter a filename for the disk image, add tags if necessary, then choose where to save it.
This is the name that appears in the Finder, where you save the disk image file before opening it.
In the Name field, enter the name for the disk image.
This is the name that appears on your desktop and in the Finder sidebar, after you open the disk image.
In the Size field, enter a size for the disk image.
Click the Format pop-up menu, then choose the format for the disk:
If the disk image will be used with a Mac that has a solid state drive (SSD) and uses macOS 10.13 or later, choose APFS or APFS (Case-sensitive).
If the disk image will be used with a Mac with macOS 10.12 or earlier, choose Mac OS Extended (Journaled) or Mac OS Extended (Case-sensitive, Journaled).
If the disk image will be used with a Mac or Windows computer and is 32 GB or less, choose MS-DOS (FAT); if it’s over 32 GB, choose ExFAT.
To encrypt the disk image, click the Encryption pop-up menu, then choose an encryption option.
Click the Partitions pop-up menu, then choose a partition layout.
Click the Image Format pop-up menu, then choose an option:
Sparse bundle disk image: Same as a sparse disk image (below), but the directory data for the image is stored differently. Uses the .sparsebundle file extension.
Sparse disk image: Creates an expandable file that shrinks and grows as needed. No additional space is used. Uses the .sparseimage file extension.
Read/write disk image: Allows you to add files to the disk image after it’s created. Uses the .dmg file extension.
DVD/CD master: Changes the size of the image to 177 MB (CD 8 cm). Uses the .cdr file extension.
Click Save, then click Done.
Disk Utility creates the disk image file where you saved it in the Finder and mounts its disk icon on your desktop and in the Finder sidebar.
In the Finder, copy your files to the mounted disk image, then eject it.
Restore the disk image to a disk.
For more information about disk image types, see the manual (man) page for hdiutil.
Create a disk image from a disk or connected device
You can create a disk image that includes the data and free space on a physical disk or connected device, such as a USB device. For example, if a USB device or volume is 80 GB with 10 GB of data, the disk image will be 80 GB in size and include data and free space. You can then restore that disk image to another volume.
In the Disk Utility app on your Mac, select a disk, volume, or connected device in the sidebar.
Choose File > New Image, then choose “Image from [device name].”
Enter a filename for the disk image, add tags if necessary, then choose where to save it.
This is the name that appears in the Finder, where you save the disk image file before opening it.
Click the Format pop-up menu, then choose an option:
Read-only: The disk image can’t be written to, and is quicker to create and open.
Compressed: Compresses data, so the disk image is smaller than the original data. The disk image is read-only.
Read/write: Allows you to add files to the disk image after it’s created.
DVD/CD master: Can be used with third-party apps. It includes a copy of all sectors of the disk image, whether they’re used or not. When you use a master disk image to create other DVDs or CDs, all data is copied exactly.
To encrypt the disk image, click the Encryption pop-up menu, then choose an encryption option.
Click Save, then click Done.
Disk Utility creates the disk image file where you saved it in the Finder and mounts its disk icon on your desktop and in the Finder sidebar.
Important: Don’t create a disk image of a disk that you believe to be failing or that contains corrupted information. The disk image may not serve as a reliable backup.
For technical information about creating a restore disk image, see the Apple Software Restore (ASR) manual (man) page.
Create a disk image from a folder or connected device
You can create a disk image that contains the contents of a folder or connected device, such as a USB device. This method doesn’t copy a device’s free space to the disk image. For example, if a USB device or volume is 80 GB with 10 GB of data, the disk image will be 10 GB in size and include only data, not free space. You can then restore that disk image to another volume.
In the Disk Utility app on your Mac, choose File > New Image, then choose Image from Folder.
Select the folder or connected device in the dialog that appears, then click Open.
Enter a filename for the disk image, add tags if necessary, then choose where to save it.
This is the name that appears in the Finder, where you save the disk image file before opening it.
To encrypt the disk image, click the Encryption pop-up menu, then choose an encryption option.
Click the Image Format pop-up menu, then choose an option:
Read-only: The disk image can’t be written to, and is quicker to create and open.
Compressed: Compresses data, so the disk image is smaller than the original data. The disk image is read-only.
Read/write: Allows you to add files to the disk image after it’s created.
DVD/CD master: Can be used with third-party apps. It includes a copy of all sectors of the disk image, whether they’re used or not. When you use a master disk image to create other DVDs or CDs, all data is copied exactly.
Hybrid image (HFS+/ISO/UDF): This disk image is a combination of disk image formats and can be used with different file system standards, such as HFS, ISO, and UDF.
Click Save, then click Done.
Disk Utility creates the disk image file where you saved it in the Finder and mounts its disk icon on your desktop and in the Finder sidebar.
For technical information about creating a restore disk image, see the Apple Software Restore (ASR) manual (man) page.
Create a secure disk image
If you have confidential documents that you don’t want others to see without your permission, you can put them in an encrypted disk image.
Note: If you want to protect the contents of the system disk, turn on FileVault using the FileVault pane of Security & Privacy Preferences.
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In the Disk Utility app on your Mac, choose File > New Image > Blank Image.
Enter a filename for the disk image, add tags if necessary, then choose where to save it.
This is the name that appears in the Finder, where you save the disk image file before opening it.
In the Name field, enter the name for the disk image.
This is the name that appears on your desktop and in the Finder sidebar, after you open the disk image.
In the Size field, enter a size for the disk image.
Click the Format pop-up menu, then choose a format:
If you’re using the encrypted disk image with a Mac computer using macOS 10.13 or later, choose APFS or APFS (Case-sensitive).
If you’re using the encrypted disk image with a Mac computer using macOS 10.12 or earlier, choose Mac OS Extended (Journaled) or Mac OS Extended (Case-sensitive, Journaled).
Click the Encryption pop-up menu, then choose an encryption option.
Enter and re-enter a password to unlock the disk image, then click Choose.
WARNING: If you forget this password, you won’t be able to open the disk image and view any of the files.
Use the default settings for the rest of the options:
Click the Partitions pop-up menu, then choose Single partition - GUID Partition Map.
Click the Image Format pop-up menu, then choose “read/write” disk image.
Click Save, then click Done.
Disk Utility creates the disk image file where you saved it in the Finder and mounts its disk icon on your desktop and in the Finder sidebar.
In the Finder , copy the documents you want to protect to the disk image.
If you want to erase the original documents so they can’t be recovered, drag them to the Trash, then choose Finder > Empty Trash.
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When you’re finished using the documents on the secure disk image, be sure to eject the disk image. As long as it’s available on your desktop, anyone with access to your computer can use the documents on it.
To access the data in a disk image, double-click it. It appears on your desktop, and you can add, remove, and edit files on it just as you would with a disk.
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