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When creating a disk image (.cdr or .dmg) from a physical CD or DVD on Mac (using Disk Utility > New Image) one can choose between 'CD\DVD Master‘ and 'compressed'.

Apart from the obvious fact that a compressed image will be smaller, are there other differences? Will a compressed image for example not expand to an exact copy of the CD or DVD?

  • That's the idea! I'd imagine "Master" would be better if you planned to create more, simply because you wouldn't have to decompress it to burn it. – Phoshi Oct 2 '09 at 20:44
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Master is uncompressed and does not need to be expanded to be restored. A compressed image is obviously smaller, but must be uncompressed before it can be manipulated. If performance is a concern in restoring the image then use a Master. If you can live with a little longer time, but a smaller footprint, go with compressed. Both will end up with the same result.

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The accepted answer's claim that the compressed image will be slower to restore is likely wrong (unless you have an SSD): with current CPUs, decompression is faster than reading more data from a hard disk. As a case in point, all applications of Snow Leopard are stored compressed to decrease launch time.

The end result is the same with all disk image formats (that's the very definition of a disk image...), but the CD/DVD master is more interoperable, while the compressed format is likely smaller and has the added advantage of a checksum.

The help of Disk Utility has this to say on the subject “Disk image formats”:

compressed: Choose this if you don’t need to add more files to the image and you want to conserve space. The data is compressed so the disk image is smaller than the original data.

CD/DVD master: Choose this if you want to use the disk image with a third-party application. The disk image contains a copy of all sectors of the disk, whether they’re used or not, and copies them to other CDs or DVDs bit for bit.

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The .dmg format is Mac OS X–only. A compressed .dmg disk image will have a checksum so that you can verify that the file is not damaged.

If you make a master (.cdr/.iso) image, it can also be burned using other operating systems. And, depending on the filesystem of the CD/DVD, you might even be able to mount the master image.

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