Disk Utility (Yosemite): Restore a disk
You can restore a disk to another disk, partition, or external disk connected to your computer. When you restore a disk to another disk, it makes an exact copy of the disk including any partitions you may have had on the original disk to the destination disk. For example, you can restore your startup disk to an external disk to use as a backup.
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Select the disk in the sidebar you want to restore, then click Restore.
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Drag the disk you want to restore to the Source field.
You can drag it from the list at the left or from the Finder sidebar.
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Drag the disk that will contain the copy to the Destination field.
If you can’t drag a particular disk to that field, you can’t copy to it. If you’re trying to restore your startup disk to another disk, you must start up your computer from a different disk. All the contents of the destination disk will be erased.
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Click the Restore button.
You can also use the Disk Utility in OS X Recovery to restore a disk. For information, see Apple OS X Recovery.