A device image is a special kind of disk image that is an exact, block-level
copy of the source volume and its filesystem, including features such as hard
links and compressed system files. You can use device images to:
- Create a complete backup of your Mac. Device images support
compression, so you can make the most efficient use of your storage, perhaps
keeping multiple full backups on the same drive. They also support
encryption, so that you can store your backup off-site (thus protecting
against fire, theft, etc.) without worrying about anyone else accessing your
sensitive files.
- Create a reusable image of a Mac, with everything installed and
configured the way you want, then clone it onto multiple Macs, e.g.
in a computer lab, so that they are all set up identically.
Device images have several advantages over other formats:
- Device images are generally faster to create than non-device images
containing the same files.
- You can back up a bootable hard drive, then restore the backup, and
the restored drive will be bootable.
- You can restore a device image to a hard drive without any special
software. Just boot from the installation DVD that came with your Mac and
choose Disk Utility from the Utilities menu.
- Device images efficiently store linked files, so they’re great for
archiving old Time Machine
backups.
- You can use DropDMG’s Verify Image… command to check that the
backup device is not damaged and that you’ll be able to restore the
files.
Since a device image is an exact copy of a drive, DropDMG has to make sure that
the drive contents don’t change while it’s making the image. In order to do
this, it unmounts the drive, copies the data, and then remounts the drive.
The startup disk is always in use and thus cannot be unmounted. To create a
device image of your startup disk there are several options:
- Boot your Mac from an external hard drive. (It’s always a good idea
to have a bootable backup drive, anyway.)
- Run DropDMG on another Mac and use Target Disk Mode to connect your Mac as a
hard disk.
- Use a utility such as SuperDuper to clone your boot
drive to backup drive, and then use DropDMG to create a device image
of the backup drive. SuperDuper’s Smart Update feature can quickly
bring an on-site clone drive up-to-date. Then you can use DropDMG to
create an encrypted device image of the clone for off-site storage.
To create a device image of a hard drive:
- Go to the Configurations tab of DropDMG’s Settings window.
- Select one of the compressed .dmg formats.
- For an encrypted backup, choose an encryption level and set a
passphrase. Encryption is highly recommended if you will be storing
your backup off-site.
- (Optional) Make sure that Optimize for restores is checked. This can make
backing up much slower, but it makes restoring faster.
- Drag the hard drive onto the DropDMG window or use one of the
other methods.
The Restoring Files and Disks section describes how to restore your backup.
APFS
Due to limitations of macOS 10.13:
- Device images of APFS volumes include all the volumes in the APFS
container. The system does not support device images of individual APFS
volumes.
- Device images of APFS volumes cannot be mounted to access their files
directly. The system will report a “no mountable file systems” error.
However, you can still use Disk Utility to restore the disk image.
If you just want to a create a regular (non-device) image containing all the
files and folders on a volume, you can use CreateDeviceImages in the
esoteric preferences to tell DropDMG not to create device images.