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From Apple Help

Creating a disk image of a device, folder, or volume

You can use Disk Utility to create an image of a single device, folder, or volume. You can use the disk image to transfer files from one computer to another, or burn the image on a CD or DVD disc that you can use to restore the contents of another disk.

If you want to create a disk image of files that are in different locations, gather them into a single folder or onto a single volume before you begin.

You can create a full disk image that uses the same amount of disk space as the disk it represents, or create a sparse disk image that includes only the data on the disk. For example, a 10MB disk may have only 5MB of data. A full disk image would be 10MB, while a sparse disk image would be 5MB.

Open Disk Utility, located in Applications/Utilities.
If you're creating a disk image from a folder, choose Images > New > "Image from Folder." Then select the folder you want from the navigation window.

If you're creating a disk image from a device or volume, select it in the list on the left side of the Disk Utility window, then choose Image > New > "Image from [disk or volume name]."
Type a name for the image, and choose a disk format.
Choose Encryption, if desired, then click Save.

The name of the disk image is followed by the filename extension ".dmg.
 
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