What is a disk image??

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Level 1

99 points

What is a disk image??

Hello,

I've just backed up my Mac, onto an external hard disk. I've now begun to think that I should make it secure, so if it ever fell into the wrong hands they can't access it without a password.

I understand that I have to create a 'Disk Image'. But what is a Disk Image? Is it simply a folder with my backup in, which is protected by a password?

I've opened Disk Utility, gone to File > New > Disk Image from Folder > selected my backup folder on the external hard disk > selected encription > and saved. It is now taking an age to make a Disk Image (whatever that is).

Once it has finished, do I treat it like a normal folder? Does it move the original backup into the Disk Image, or should I delete this? How do I use it?

Thanks

2.4 GHz Intel iMac, Mac OS X (10.5.3), 10.5.4

Posted on Aug 21, 2008 8:13 AM

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Answer 1

Aug 21, 2008 11:38 AM in response to Mark Epton

AFAIK, a disk image is a single file that contains all the folders and files that were on the original disk (or selected part of the original disk).
It could be a copy of your entire hard disk, a CD/DVD, or an application.

Sounds like what you've done is made an image of an existing backup folder, rather than a new image of your HDD. It'll contain everything that was in your backup folder, but should take up less space. What you do with it (and the original) depends on what application you used to make the original backup folder.

If you want to make a new image to keep as a backup, highlight your HDD/boot partition (normally labelled Macintosh HD) in Disk Utility, then select file>New>Disk Image From (device). This will create an image that's an exact copy of the contents of your boot partition.

Once the image is created, you can treat it like any other file.

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Answer 2

AJ

Level 5

7,075 points

Aug 21, 2008 1:59 PM in response to Mark Epton

a disk image is kind of like an eletronic version of a physical disk.
(Many programs you download from the web are in a disk image format.)

Anyway...when you create a disk image, the resultant file is called whatever.dmg.

When you 2x click whatever.dmg file, you'll notice a new icon on your desktop named "whatever". it looks like an old external floppy drive.

You copy however many files/folders you want to the whatever icon.
(Remember, the system is treating it like a separate physical disk.)

When you are done, you drag the whatever icon to the trash (same as ejecting a physical disk). The icon disappears, but the file whatever.dmg remains.

All the files you copied to the new icon are now stored inside the single .dmg file.

2x click whatever.dmg, it opens an all your files are there.

you can add/remove files from the dmg file, store it on an external drive, etc.

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Answer 3

Mark Epton Author

Level 1

99 points

Aug 22, 2008 1:57 AM in response to clockworks

Thanks for response.

I want to backup my entire computer onto an external hard disk. I've done as you said: made a Disk Image of my Hard Disk (password protected, read/write). I can now copy that to my external hard disk.

I now intend to backup my Users > Home folder every week. But I can't drag my Home folder onto the .dmg file to copy and replace the old Home folder.

Does this mean that I have to double click the .dmg file, to get the disk icon to appear on my desktop. I then drag my Home folder to this folder to copy and replace the old Home folder. And amazingly this links to and updates the .dmg file.

Won't this take ages? The .dmg file has to open (process 1), the file has to copy and replace the old Home folder (process 2) and then somehow the disk icon on the desktop has to link back to the .dmg file (process 3)

Can I not just password protect the entire external hard disk or password protect a folder?

Thanks

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Answer 4

Mark Epton Author

Level 1

99 points

Aug 22, 2008 2:05 AM in response to clockworks

One more thing! I think I'm thinking aloud here...

Is it possible/easy to make a Disk Image of my entire hard disk, apart from my Home folder?

This way I back up everything as a Disk Image and then backup my Home folder as a separate Disk Image. Then every week I can delete the old Home.dmg file and create a new one - without touching the bigger .dmg file.

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Answer 5

Aug 22, 2008 10:23 AM in response to Mark Epton

It does sound like a complicated way of doing it. Why not just use a backup application to do it for you?

Time Machine will do automatic backups, just copying files that have changed, although it needs it's own partition on the external disk.
Carbon Copy Cloner can do a similar job (full or incremental backups to a partition), or it can work with disk images.

I use both apps to 2 different external disks. The advantage of using CCC to clone to a partition (rather than an image) is you can boot from it if your internal HDD fails.

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