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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:

- 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 FireWire 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:

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.

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