Once you’ve backed up the contents of a hard disk or CD-ROM to a disk image, how can you restore the files?
If You Just Need to Access Specific Files
Double-click the .dmg file to mount it, then locate the files using the Finder. If a large disk image is split into multiple .dmg and .dmgpart files, move or copy them all into a single folder before trying to mount the .dmg.
If you have many .dmg files, you can use a utility such as CDFinder to catalog them so that you can easily find which .dmg contains the files that you’re looking for.
To Restore an Entire Drive
Open the Disk Utility program in the Utilities folder of the Applications folder.
You can also access Disk Utility by booting from your macOS installation CD/DVD or recovery partition; that way, you can restore your drive without first having to install macOS.
You can also restore disk images using the asr command-line tool.
Choose Scan Image for Restore… from Disk Utility’s Images menu and select the disk image file. If you used DropDMG’s Optimize for restores option when creating the backup, you can skip this step.
Select the target drive in the list at the left side of the main Disk Utility window.
Note: With macOS 10.7 or later, the target drive must be at least as large as the source drive—the full capacity of the source drive, even if some of this is free space. This is because Disk Utility does an exact block copy, so everything has to line up (including free space). If your target drive is too small, you may be able to get around this by using DropDMG to convert your disk image to read-write format and then using Disk Utility’s Resize Image command to make it smaller (by getting rid of the free space).
macOS 10.12 and later: Click on the Restore button in the toolbar. Click the Image… button and choose your disk image file.
macOS 10.11 and earlier: Click on the Restore tab. Drag the image file onto the Source field, and then drag the target disk from the list to the Destination field.
When you click the Restore button, Disk Utility will erase the target drive and replace its contents with an exact copy of the drive that the image was created from. If the original drive was bootable, the restored drive will be, too.
To Restore an Entire CD or DVD
Choose Burn… from the File menu and select the disk image file. Make sure that Burn contents of disk images is selected. This lets you create a copy of the original disc.