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3.2   Making a Bootable Device Image of a Hard Drive

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:

Device images have several advantages over other formats:

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:

To create a device image of a hard drive:

  1. Go to the Configurations tab of DropDMG’s Preferences window.
  2. Select one of the compressed .dmg formats.
  3. 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.
  4. (Optional) Make sure that Optimize for restores is checked. This can make backing up much slower, but it makes restoring faster.
  5. 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.


Due to limitations of macOS 10.13:

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.

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