I have a .DMG file of around 7GB.

When I try and open it using Ultra ISO or Magic ISO it just sees it as a 986MB file?

I would love to burn this to a USB key or something, but obviously it wont boot at the moment as it's missing the essential files (is it partitioned?)

Can anyone shed some light? Am using Windows 7.

  • Can those tools see everything inside a dmg file? Mount the image on a Mac to see what's really inside.
    – afrazier
    Jan 28, 2011 at 22:45

2 Answers 2


Disk Images can be larger than what they actually contain.

Think of a CD-ROM with a capacity of 650-800 MB: You can burn a 4kb text file on them and be done with it.


It's a sparse image, as I'm unaware of any DVD software for Windows aware of this (but do double check) you have to burn it on a mac, or at least turn it into an iso. And I think you have to bless it (either for OSX to run, or for a Mac to run it), which is Mac only as well.

  • Sparse images only take up as much space as the data within it is using. I think the op is saying that the file is 7 gb but the contents are less than 1 gb.
    – afrazier
    Jan 28, 2011 at 22:44
  • Sparse images have a corresponding file extension anyway.
    – Daniel Beck
    Jan 28, 2011 at 22:53

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