Setting a hard drive as an image?

No offense, but you clearly have no idea what a disk image is. A disk image is a file that, when launched, presents it's contents to the computer as if a drive has been connected. Making a drive, when connected, "mount" as a disk image makes no sense. There is no such thing.

However, it sounds like what you want to do is create a disk image of your external drive (keep in mind you can't boot from a disk image, you can only then create a duplicate copy onto a DVD or another drive and boot from that), and if that's the case, you'll use Disk Utility. Select the drive in Disk Utility (the volume by it's name, btw, not the physical drive that lists the size), click New Image in the toolbar, and inspect the various options listed.

jW

 

Your hard drive will never mount as a DMG, because it's NOT a DMG, it's an actual, physical hard drive. Now, if you want to make a DMG that is a clone of the your external hard drive, that you can do. Mount the drive as normal, go into to Disk Utility, and highlight the volume you want to image. Select File > New > Image from (disk path/name).

 

Oh.

Open Disk Utility, go to the "Restore" tab, drag your external drive (the volume, not the drive itself) into the "Destination" field, then click on the "Image" button for the "Source" field, and select your disk image, then click the "Restore" button.