Disk Utility has a great feature that can be used to copy the content of one volume over to another. This way users have an exact copy of the volume or image file, often used for backup or archiving purposes. The process is similar to cloning a volume and the good thing about it is that the clone can be used as a Mac's startup drive.
Dubbed as “Restore”, the Disk Utility feature has gone through some changes since its inception, the most noticeable being Apple’s move away from legacy HFS+ file system to APFS with the release of High Sierra.
The restore function needed to create a clone of the startup drive, but this is also required to create a copy of any image or volume that can be mounted on a Mac. This function will copy the source volume or image file over to the destination volume, so users need a disk with enough storage space to adopt a new volume large enough to hold the data from the source volume.
For the cloning or restoring process to work, the Mac needs to mount both the source and destination volumes. Restoring from image files requires an additional step, which we will detail further down the article.
Creating a Startup Clone
Prior to macOS High Sierra you couldn't just clone a startup drive directly for one very simple reason: the volume the Mac booted from couldn't be unmounted during the restore process. That's why cloning a startup drive either requires entering macOS Recovery or booting the Mac from another drive containing macOS. To do so, follow these steps:
- Launch Disk Utility and select the destination volume that you want the data to be copied to.
- With the destination volume selected, click on the “Restore” button.
- Select the source volume from the drop-down menu next to the “Restore from” option. Important: The selected destination volume is going to be erased in the next step, so make the choice wisely.
- Click the “Restore” button.
- Wait until the cloning process is finished, then press the “Done” button.
Note that cloning a startup disk requires a dedicated external drive because it will erase all of the destination drive’s content and replace it with what you have on your internal drive.
Create an Image File
Sometimes you only want some of the data stored on a Mac to be made portable. For that or any other reason, you’ll want to use the “New Image” option in Disk Utility, which will result in a DMG file of the chosen folder or even an entire hard drive. To create a DMG file, follow the steps below:
- Launch Disk Utility.
- Click on File > New Image.
- Depending on your need, select the “Image from folder” or “Image from [Disk drive]”.
- Select the destination and image format (compressed).
- Click “Save”.
When you create a disk image of a selected folder or disk it's important to make sure that the data was copied correctly. Fortunately, macOS helps here with the “Add checksum” function, which can be added to a disk image. Checksum is a calculation of all the data on a disk or disk image. To add it:
- Launch Disk Utility > Images.
- Choose “Add Checksum” and select the disk image file to add the checksum to.
- Click “Open”, followed by “Done”.
If you don't restore the data on the Mac immediately – as you would during a troubleshooting process – then you will need to make sure it is still intact when, for example, it comes to unpacking a year-old disk image. Data can become corrupted while being copied or if someone modifies it and so to verify the integrity of the data, all that needs to be done is to verify its checksum. To do that, follow these steps:
- Launch Disk Utility > Images > Verify.
- Select the disk image you want the software to check.
- Click “Verify” followed by “Done”.
- Let the utility finish its job. If successfully, you are ready to restore the data from the image file.
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