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When you download a hunk of Mac software, it sometimes arrives in the form of a disk image, which is identified by its .dmg extension. Disk Utility has the ability to create disk images, which can come in handy but how so? Well let's follow along. One of the most immediately useful things you can do with disk images is copy a mounted volume or folder on your Mac. For example, suppose you just got a CD ROM based game for your kid, and that kid is likely to smear it with peanut butter and honey, meaning you have to buy another copy of the disk.
You can create a disk image of that CD and mount it; as far as the Mac is concerned the mounted disk image is the CD so you can safely tuck that disk away from your kid while still allowing him to use it. To this kind of thing insert a CD and select it in Disk Utility, choose File, New, and then Disk Image From and this will be the name of your media drive. In the sheet that appears you have a number of options. You can leave it as compressed, if you are copying a disk and you want to do it bit by bit for example an installation disk of some kind, you might choose DVD/CD Master, this is what I do.
You can also do Read Only, let's try Read Only, and encryption should be none. Click Save, and we will create a disk image on the desktop and now we have our disk image. Now this can take a while, we sped this up so you wouldn't have to sit through the creation process but it maybe a good idea if you particularly have a DVD that you are creating an image from. Start the process, go have a cup of coffee and by the time you come back everything should be ready to go.
Once you have this disk image, you can mount it simply by selecting it in Disk Utility and clicking Open. And here it is. And we drag that into the trash to unmount it or you can simply double click on the image on the Finder. I will skip that, and here it is again. As I said you can also create a disk image from a folder, to do so, choose File, New, Disk Image From Folder, navigate to our Documents folder, let's take this Music folder here and we click Image.
Now here's something interesting that you can do. In the sheet that appears choose Compressed and then one of the encryption options and click Save. You will be asked for a password for this disk image and click OK. Great, so now we have our image, so what good is this? You have now created a compressed image file that requires a password to open.
You can now pass this file along to someone you trust without fears that other people will be able to open it or don't pass it along and keep it as a password protected archive that only you can open. Speaking of password protection, you can use this disk image capability to create password protected virtual and expandable volumes on your Mac and you do it like this. Click the New Image button at the top of the window. In the sheet that appears, enter a name for your archive. In the Volume Size pop up menu, choose the size of the archive you would like, we will make this one 500 megabytes.
From the Encryption pop-up menu choose the kind of encryption that you would like, and from the Image Format pop-up menu choose Sparse Disk Image. Click Create and once again I am prompted for a password so I will enter one. Now note I am going to disable the option to remember password in my Keychain and the reason I do this is because I want this to be encrypted. If somebody sits down on my computer while I am using it if I have wandered away for a bit they come in and they find my image and double click on it, they will be prompted for a password.
If I remember that password in the Keychain this doesn't help me very much if I leave the computer and somebody double clicks on it because up comes the image, because it is in the Keychain and they can access my files, and now I will click OK to create that disk image, and there we are. So at this point I can drag files into this disk image if I like. Once I am finished with it I just drag it into the trash. Now somebody walks up to my Mac later and says, "Oh, your files, let's just check that out" and they double click on it, they are prompted for the password that I have put into the disk image.
Well let's see if we just do share password, how about that? Okay, gee, I still have to enter the password so you can see how secure this is and that's a real benefit to you. Another benefit of this sparse image is that it's expandable. Regular disk images hog all the space you have allotted to them so that if you have created 500 MB image for example, that's how much space it will take up on your hard drive. A sparse image understands that you can have as much space as you have given it if needed but it will tell the Mac OS that it is only as big as the sum of the files in it, and of course thanks to the encryption option, you now have that level of extra protection.
And that's how you create disk images in Disk Utility, up next, burning images.
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